I'd like to allow the speech synthesizer to play on the device speaker while simultaneously mixing with a phone call. I've worked with a number of different configurations but am unable to find a configuration that achieves the functionality I am trying to achieve - or allows mixing with a phone call at all. There is a flag: mixToTelephonyUplink that seems to suggest that at least some mixing with a phone call is possible using the speech synthesizer, but I'm currently unable to find almost any documentation about this flag besides basic API docs. I've had some some luck at least getting the synthesizer to always play to the speaker with the following audio session configuration - but the sound never is mixed with a phone call. Instead, it is ducked and muted while the phone call takes place. I've tried quite a few configuration combinations for the category and overrides, but nothings seems to work quite as I'd expect it to. synthesizer.mixToTelephonyUplink = true try? audioSession.setCategory(.playback, mode: .voicePrompt, options: [.mixWithOthers, .defaultToSpeaker]) try? audioSession.setActive(true, options: []) try? audioSession.overrideOutputAudioPort(.speaker) Is there some kind of documentation for this that's off the beaten path that I'm somehow missing? I'm going to continue with guess and check, but I'm starting to think this flag - and the functionality it implies, actually wasn't ever fully implemented.