App doesn't route audio to headphones (initially)

I have a VOIP app implemented using the Sinch SDK and CallKit. Everything works fine, apart from when the device has headphones plugged in. In the latter case, when the call starts, audio is still routed through the main speaker of the device. If I unplug and plug the headphones back in - during the call -, audio is then correctly routed to the headphones.

All I am doing is

func provider(_ provider: CXProvider, perform action: CXAnswerCallAction) {
    guard let c = self.currentCall else {
        action.fail()
        return
    }

    c.answer()

    self.communicationClient.audioController().configureAudioSessionForCallKitCall()
    action.fulfill()
}

Shouldn't this be taken care automatically by the OS?

from App doesn't route audio to headphones (initially)