How to select iPhone audio device before placing a call?
Before placing the call, you can open Control Center and tap the AirPlay icon to select a Bluetooth destination such as for example a pir of AirPods. The following call will then use that audio device by default.
After the call is done, you'll probably to tap the AirPlay icon again to choose iPhone as your destination.
If you want to use wired headphones, you'll obviously need to actually connect them to the phone before they can be used.
There are no other settings or functionality for doing a once-only rerouting of phone-only audio to a specific device before placing a call.
There is however a different way to overcome the problem that the calls connects before you have managed to switch the audio destination: You can add a pause or wait to the actual phone number.
I.e. when you edit a contact on your phone, you can set their phone number. If you click the button labelled &quot;+*#&quot; you get the option to place a pause or a wait. Add this to the start of the number, and you'll get additional time to switch the audio destination.
A pause is literally just a pause for 2-3 seconds (afair), whereas wait means that the phone does not dial the rest of the number until you have tapped the phone to acknowledge.