diff --git a/Signal/src/call/CallService.swift b/Signal/src/call/CallService.swift
index 1991e4691..f882f154d 100644
--- a/Signal/src/call/CallService.swift
+++ b/Signal/src/call/CallService.swift
@@ -309,7 +309,9 @@ protocol CallServiceObserver: class {
                                 in: thread)
         callRecord.save()
 
-        self.callUIAdapter.reportMissedCall(call)
+        DispatchQueue.main.async {
+            self.callUIAdapter.reportMissedCall(call)
+        }
     }
 
     /**
@@ -514,7 +516,9 @@ protocol CallServiceObserver: class {
             call.state = .remoteRinging
         case .answering:
             call.state = .localRinging
-            self.callUIAdapter.reportIncomingCall(call, thread: thread)
+            DispatchQueue.main.async {
+                self.callUIAdapter.reportIncomingCall(call, thread: thread)
+            }
             // cancel connection timeout
             self.fulfillCallConnectedPromise?()
         case .remoteRinging:
@@ -552,7 +556,9 @@ protocol CallServiceObserver: class {
 
         call.state = .remoteHangup
         // Notify UI
-        callUIAdapter.remoteDidHangupCall(call)
+        DispatchQueue.main.async {
+            self.callUIAdapter.remoteDidHangupCall(call)
+        }
 
         // self.call is nil'd in `terminateCall`, so it's important we update it's state *before* calling `terminateCall`
         terminateCall()
@@ -821,7 +827,9 @@ protocol CallServiceObserver: class {
                 return
             }
 
-            callUIAdapter.recipientAcceptedCall(call)
+            DispatchQueue.main.async {
+                self.callUIAdapter.recipientAcceptedCall(call)
+            }
             handleConnectedCall(call)
 
         } else if message.hasHangup() {
@@ -958,7 +966,9 @@ protocol CallServiceObserver: class {
             // It's essential to set call.state before terminateCall, because terminateCall nils self.call
             call.error = error
             call.state = .localFailure
-            callUIAdapter.failCall(call, error: error)
+            DispatchQueue.main.async {
+                self.callUIAdapter.failCall(call, error: error)
+            }
         } else {
             // This can happen when we receive an out of band signaling message (e.g. IceUpdate)
             // after the call has ended
@@ -1114,7 +1124,7 @@ protocol CallServiceObserver: class {
                 }
             }
         }
-        
+
         // Prevent screen from dimming during video call.
         let hasLocalOrRemoteVideo = localVideoTrack != nil || remoteVideoTrack != nil
         UIApplication.shared.isIdleTimerDisabled = hasLocalOrRemoteVideo
diff --git a/Signal/src/call/NonCallKitCallUIAdaptee.swift b/Signal/src/call/NonCallKitCallUIAdaptee.swift
index 876c9d68c..cc025565f 100644
--- a/Signal/src/call/NonCallKitCallUIAdaptee.swift
+++ b/Signal/src/call/NonCallKitCallUIAdaptee.swift
@@ -23,6 +23,8 @@ class NonCallKitCallUIAdaptee: CallUIAdaptee {
     }
 
     func startOutgoingCall(handle: String) -> SignalCall {
+        AssertIsOnMainThread()
+
         let call = SignalCall.outgoingCall(localId: UUID(), remotePhoneNumber: handle)
 
         CallService.signalingQueue.async {
@@ -37,6 +39,8 @@ class NonCallKitCallUIAdaptee: CallUIAdaptee {
     }
 
     func reportIncomingCall(_ call: SignalCall, callerName: String) {
+        AssertIsOnMainThread()
+
         Logger.debug("\(TAG) \(#function)")
 
         // present Call View controller
@@ -52,10 +56,14 @@ class NonCallKitCallUIAdaptee: CallUIAdaptee {
     }
 
     func reportMissedCall(_ call: SignalCall, callerName: String) {
+        AssertIsOnMainThread()
+
         notificationsAdapter.presentMissedCall(call, callerName: callerName)
     }
 
     func answerCall(localId: UUID) {
+        AssertIsOnMainThread()
+
         CallService.signalingQueue.async {
             guard let call = self.callService.call else {
                 assertionFailure("\(self.TAG) in \(#function) No current call.")
@@ -72,6 +80,8 @@ class NonCallKitCallUIAdaptee: CallUIAdaptee {
     }
 
     func answerCall(_ call: SignalCall) {
+        AssertIsOnMainThread()
+
         CallService.signalingQueue.async {
             guard call.localId == self.callService.call?.localId else {
                 assertionFailure("\(self.TAG) in \(#function) localId does not match current call")
@@ -84,6 +94,8 @@ class NonCallKitCallUIAdaptee: CallUIAdaptee {
     }
 
     func declineCall(localId: UUID) {
+        AssertIsOnMainThread()
+
         CallService.signalingQueue.async {
             guard let call = self.callService.call else {
                 assertionFailure("\(self.TAG) in \(#function) No current call.")
@@ -100,6 +112,8 @@ class NonCallKitCallUIAdaptee: CallUIAdaptee {
     }
 
     func declineCall(_ call: SignalCall) {
+        AssertIsOnMainThread()
+
         CallService.signalingQueue.async {
             guard call.localId == self.callService.call?.localId else {
                 assertionFailure("\(self.TAG) in \(#function) localId does not match current call")
@@ -111,10 +125,14 @@ class NonCallKitCallUIAdaptee: CallUIAdaptee {
     }
 
     func recipientAcceptedCall(_ call: SignalCall) {
+        AssertIsOnMainThread()
+
         PeerConnectionClient.startAudioSession()
     }
 
     func localHangupCall(_ call: SignalCall) {
+        AssertIsOnMainThread()
+
         CallService.signalingQueue.async {
             // If both parties hang up at the same moment,
             // call might already be nil.
@@ -128,14 +146,20 @@ class NonCallKitCallUIAdaptee: CallUIAdaptee {
     }
 
     internal func remoteDidHangupCall(_ call: SignalCall) {
+        AssertIsOnMainThread()
+
         Logger.debug("\(TAG) in \(#function) is no-op")
     }
 
     internal func failCall(_ call: SignalCall, error: CallError) {
+        AssertIsOnMainThread()
+
         Logger.debug("\(TAG) in \(#function) is no-op")
     }
 
     func setIsMuted(call: SignalCall, isMuted: Bool) {
+        AssertIsOnMainThread()
+
         CallService.signalingQueue.async {
             guard call.localId == self.callService.call?.localId else {
                 assertionFailure("\(self.TAG) in \(#function) localId does not match current call")
@@ -147,6 +171,8 @@ class NonCallKitCallUIAdaptee: CallUIAdaptee {
     }
 
     func setHasLocalVideo(call: SignalCall, hasLocalVideo: Bool) {
+        AssertIsOnMainThread()
+
         CallService.signalingQueue.async {
             guard call.localId == self.callService.call?.localId else {
                 assertionFailure("\(self.TAG) in \(#function) localId does not match current call")
diff --git a/Signal/src/call/Speakerbox/CallKitCallUIAdaptee.swift b/Signal/src/call/Speakerbox/CallKitCallUIAdaptee.swift
index e4599a29e..f427d6032 100644
--- a/Signal/src/call/Speakerbox/CallKitCallUIAdaptee.swift
+++ b/Signal/src/call/Speakerbox/CallKitCallUIAdaptee.swift
@@ -49,6 +49,8 @@ final class CallKitCallUIAdaptee: NSObject, CallUIAdaptee, CXProviderDelegate {
     }
 
     init(callService: CallService, notificationsAdapter: CallNotificationsAdapter) {
+        AssertIsOnMainThread()
+
         self.callManager = CallKitCallManager()
         self.callService = callService
         self.notificationsAdapter = notificationsAdapter
@@ -62,6 +64,8 @@ final class CallKitCallUIAdaptee: NSObject, CallUIAdaptee, CXProviderDelegate {
     // MARK: CallUIAdaptee
 
     func startOutgoingCall(handle: String) -> SignalCall {
+        AssertIsOnMainThread()
+
         let call = SignalCall.outgoingCall(localId: UUID(), remotePhoneNumber: handle)
 
         // Add the new outgoing call to the app's list of calls.
@@ -74,11 +78,15 @@ final class CallKitCallUIAdaptee: NSObject, CallUIAdaptee, CXProviderDelegate {
 
     // Called from CallService after call has ended to clean up any remaining CallKit call state.
     func failCall(_ call: SignalCall, error: CallError) {
+        AssertIsOnMainThread()
+
         provider.reportCall(with: call.localId, endedAt: Date(), reason: CXCallEndedReason.failed)
         self.callManager.removeCall(call)
     }
 
     func reportIncomingCall(_ call: SignalCall, callerName: String) {
+        AssertIsOnMainThread()
+
         // Construct a CXCallUpdate describing the incoming call, including the caller.
         let update = CXCallUpdate()
         update.remoteHandle = CXHandle(type: .phoneNumber, value: call.remotePhoneNumber)
@@ -104,39 +112,57 @@ final class CallKitCallUIAdaptee: NSObject, CallUIAdaptee, CXProviderDelegate {
     }
 
     func answerCall(localId: UUID) {
+        AssertIsOnMainThread()
+
         assertionFailure("CallKit should answer calls via system call screen, not via notifications.")
     }
 
     func answerCall(_ call: SignalCall) {
+        AssertIsOnMainThread()
+
         callManager.answer(call: call)
     }
 
     func declineCall(localId: UUID) {
+        AssertIsOnMainThread()
+
         assertionFailure("CallKit should decline calls via system call screen, not via notifications.")
     }
 
     func declineCall(_ call: SignalCall) {
+        AssertIsOnMainThread()
+
         callManager.localHangup(call: call)
     }
 
     func recipientAcceptedCall(_ call: SignalCall) {
+        AssertIsOnMainThread()
+
         // no - op
         // TODO provider update call connected?
     }
 
     func localHangupCall(_ call: SignalCall) {
+        AssertIsOnMainThread()
+
         callManager.localHangup(call: call)
     }
 
     func remoteDidHangupCall(_ call: SignalCall) {
+        AssertIsOnMainThread()
+
         provider.reportCall(with: call.localId, endedAt: nil, reason: CXCallEndedReason.remoteEnded)
     }
 
     func setIsMuted(call: SignalCall, isMuted: Bool) {
+        AssertIsOnMainThread()
+
         callManager.setIsMuted(call: call, isMuted: isMuted)
     }
 
     func setHasLocalVideo(call: SignalCall, hasLocalVideo: Bool) {
+        AssertIsOnMainThread()
+
         let update = CXCallUpdate()
         update.remoteHandle = CXHandle(type: .phoneNumber, value: call.remotePhoneNumber)
         update.hasVideo = hasLocalVideo
@@ -152,6 +178,8 @@ final class CallKitCallUIAdaptee: NSObject, CallUIAdaptee, CXProviderDelegate {
     // MARK: CXProviderDelegate
 
     func providerDidReset(_ provider: CXProvider) {
+        AssertIsOnMainThread()
+
         Logger.debug("\(TAG) in \(#function)")
 
         // TODO
@@ -174,6 +202,8 @@ final class CallKitCallUIAdaptee: NSObject, CallUIAdaptee, CXProviderDelegate {
     }
 
     func provider(_ provider: CXProvider, perform action: CXStartCallAction) {
+        AssertIsOnMainThread()
+
         Logger.debug("\(TAG) in \(#function) CXStartCallAction")
 
         // TODO does this work when `action.handle.value` is not in e164 format, e.g. if called via intent?
@@ -207,6 +237,8 @@ final class CallKitCallUIAdaptee: NSObject, CallUIAdaptee, CXProviderDelegate {
     }
 
     func provider(_ provider: CXProvider, perform action: CXAnswerCallAction) {
+        AssertIsOnMainThread()
+
         Logger.debug("\(TAG) Received \(#function) CXAnswerCallAction")
         // Retrieve the SpeakerboxCall instance corresponding to the action's call UUID
         guard let call = callManager.callWithLocalId(action.callUUID) else {
@@ -232,6 +264,8 @@ final class CallKitCallUIAdaptee: NSObject, CallUIAdaptee, CXProviderDelegate {
     }
 
     public func provider(_ provider: CXProvider, perform action: CXEndCallAction) {
+        AssertIsOnMainThread()
+
         Logger.debug("\(TAG) Received \(#function) CXEndCallAction")
         guard let call = callManager.callWithLocalId(action.callUUID) else {
             action.fail()
@@ -257,6 +291,8 @@ final class CallKitCallUIAdaptee: NSObject, CallUIAdaptee, CXProviderDelegate {
     }
 
     public func provider(_ provider: CXProvider, perform action: CXSetHeldCallAction) {
+        AssertIsOnMainThread()
+
         Logger.debug("\(TAG) Received \(#function) CXSetHeldCallAction")
         guard let call = callManager.callWithLocalId(action.callUUID) else {
             action.fail()
@@ -282,6 +318,8 @@ final class CallKitCallUIAdaptee: NSObject, CallUIAdaptee, CXProviderDelegate {
     }
 
     public func provider(_ provider: CXProvider, perform action: CXSetMutedCallAction) {
+        AssertIsOnMainThread()
+
         Logger.debug("\(TAG) Received \(#function) CXSetMutedCallAction")
         guard callManager.callWithLocalId(action.callUUID) != nil else {
             Logger.error("\(TAG) Failing CXSetMutedCallAction for unknown call: \(action.callUUID)")
@@ -296,20 +334,28 @@ final class CallKitCallUIAdaptee: NSObject, CallUIAdaptee, CXProviderDelegate {
     }
 
     public func provider(_ provider: CXProvider, perform action: CXSetGroupCallAction) {
+        AssertIsOnMainThread()
+
         Logger.warn("\(TAG) unimplemented \(#function) for CXSetGroupCallAction")
     }
 
     public func provider(_ provider: CXProvider, perform action: CXPlayDTMFCallAction) {
+        AssertIsOnMainThread()
+
         Logger.warn("\(TAG) unimplemented \(#function) for CXPlayDTMFCallAction")
     }
 
     func provider(_ provider: CXProvider, timedOutPerforming action: CXAction) {
+        AssertIsOnMainThread()
+
         Logger.debug("\(TAG) Timed out \(#function)")
 
         // React to the action timeout if necessary, such as showing an error UI.
     }
 
     func provider(_ provider: CXProvider, didActivate audioSession: AVAudioSession) {
+        AssertIsOnMainThread()
+
         Logger.debug("\(TAG) Received \(#function)")
 
         // Start recording
@@ -317,6 +363,8 @@ final class CallKitCallUIAdaptee: NSObject, CallUIAdaptee, CXProviderDelegate {
     }
 
     func provider(_ provider: CXProvider, didDeactivate audioSession: AVAudioSession) {
+        AssertIsOnMainThread()
+
         Logger.debug("\(TAG) Received \(#function)")
 
         /*
diff --git a/Signal/src/call/UserInterface/CallUIAdapter.swift b/Signal/src/call/UserInterface/CallUIAdapter.swift
index b448dcd82..98e581a8c 100644
--- a/Signal/src/call/UserInterface/CallUIAdapter.swift
+++ b/Signal/src/call/UserInterface/CallUIAdapter.swift
@@ -63,6 +63,8 @@ extension CallUIAdaptee {
     private let audioService: CallAudioService
 
     required init(callService: CallService, contactsManager: OWSContactsManager, notificationsAdapter: CallNotificationsAdapter) {
+        AssertIsOnMainThread()
+
         self.contactsManager = contactsManager
         if Platform.isSimulator {
             // CallKit doesn't seem entirely supported in simulator.
@@ -82,6 +84,8 @@ extension CallUIAdaptee {
     }
 
     internal func reportIncomingCall(_ call: SignalCall, thread: TSContactThread) {
+        AssertIsOnMainThread()
+
         call.addObserverAndSyncState(observer: audioService)
 
         let callerName = self.contactsManager.displayName(forPhoneIdentifier: call.remotePhoneNumber)
@@ -89,11 +93,15 @@ extension CallUIAdaptee {
     }
 
     internal func reportMissedCall(_ call: SignalCall) {
+        AssertIsOnMainThread()
+
         let callerName = self.contactsManager.displayName(forPhoneIdentifier: call.remotePhoneNumber)
         adaptee.reportMissedCall(call, callerName: callerName)
     }
 
     internal func startOutgoingCall(handle: String) -> SignalCall {
+        AssertIsOnMainThread()
+
         let call = adaptee.startOutgoingCall(handle: handle)
         call.addObserverAndSyncState(observer: audioService)
 
@@ -101,56 +109,82 @@ extension CallUIAdaptee {
     }
 
     internal func answerCall(localId: UUID) {
+        AssertIsOnMainThread()
+
         adaptee.answerCall(localId: localId)
     }
 
     internal func answerCall(_ call: SignalCall) {
+        AssertIsOnMainThread()
+
         adaptee.answerCall(call)
     }
 
     internal func declineCall(localId: UUID) {
+        AssertIsOnMainThread()
+
         adaptee.declineCall(localId: localId)
     }
 
     internal func declineCall(_ call: SignalCall) {
+        AssertIsOnMainThread()
+
         adaptee.declineCall(call)
     }
 
     internal func callBack(recipientId: String) {
+        AssertIsOnMainThread()
+
         adaptee.callBack(recipientId: recipientId)
     }
 
     internal func recipientAcceptedCall(_ call: SignalCall) {
+        AssertIsOnMainThread()
+
         adaptee.recipientAcceptedCall(call)
     }
 
     internal func remoteDidHangupCall(_ call: SignalCall) {
+        AssertIsOnMainThread()
+
         adaptee.remoteDidHangupCall(call)
     }
 
     internal func localHangupCall(_ call: SignalCall) {
+        AssertIsOnMainThread()
+
         adaptee.localHangupCall(call)
     }
 
     internal func failCall(_ call: SignalCall, error: CallError) {
+        AssertIsOnMainThread()
+
         adaptee.failCall(call, error: error)
     }
 
     internal func showCall(_ call: SignalCall) {
+        AssertIsOnMainThread()
+
         adaptee.showCall(call)
     }
 
     internal func setIsMuted(call: SignalCall, isMuted: Bool) {
+        AssertIsOnMainThread()
+
         // With CallKit, muting is handled by a CXAction, so it must go through the adaptee
         adaptee.setIsMuted(call: call, isMuted: isMuted)
     }
 
     internal func setHasLocalVideo(call: SignalCall, hasLocalVideo: Bool) {
+        AssertIsOnMainThread()
+
         adaptee.setHasLocalVideo(call: call, hasLocalVideo: hasLocalVideo)
     }
 
     internal func setIsSpeakerphoneEnabled(call: SignalCall, isEnabled: Bool) {
-        // Speakerphone is not handled by CallKit (e.g. there is no CXAction), so we handle it w/o going through the 
+        AssertIsOnMainThread()
+
+        // Speakerphone is not handled by CallKit (e.g. there is no CXAction), so we handle it w/o going through the
         // adaptee, relying on the AudioService CallObserver to put the system in a state consistent with the call's 
         // assigned property.
         CallService.signalingQueue.async {
@@ -160,6 +194,8 @@ extension CallUIAdaptee {
 
     // CallKit handles ringing state on it's own. But for non-call kit we trigger ringing start/stop manually.
     internal var hasManualRinger: Bool {
+        AssertIsOnMainThread()
+
         return adaptee.hasManualRinger
     }
 }