Here is what I see in the log:
16:33:20.236: Call is Dialing
16:33:21.088: AVAudioSessionInterruptionNotification
16:33:21.450: AVAudioSessionRouteChangeNotifica
I suspect the problem is here:
audioEngine.connect(audioTimePitch, to: mainMixer, format: mainMixer.outputFormatForBus(0))
The documentation says:
This method calls connect:to:fromBus:toBus:format: using bus 0 for the source audio node, and bus 0 for the destination audio node, except in the case of a destination which is a mixer, in which case the destination is the mixer’s nextAvailableInputBus.