How to route custom ringtone audio to speaker?

[bennnjamin]

I saw a similar comment about this here but no solution twilio/twilio-voice-ios#85 (comment).

Simply calling toggleAudioRoute(toSpeaker: true doesn't do anything during ringing with a custom ringtone. In fact, you get this error:

AVAudioSession setAggregatedIOPreference:error: Error Domain=NSOSStatusErrorDomain Code=560030580 "Operation not permitted while session has running audio I/Os" UserInfo={NSLocalizedDescription=Operation not permitted while session has running audio I/Os}

What is the correct way to allow the user to activate the speaker while the call is still ringing using a custom ringtone?