Question about Call Audio Routing

Hi!

I am wondering if it would be possible to make it so that the standard setting for audio routing for calls is set to "speaker" (I know this can be done in Settings > Accessibility > Touch > Call Audio Routing), but also at the same time make it so that if I connect a bluetooth headset, the iPhone will automatically choose the bluetooth headset as the call audio source? It works if I answer a phone call/make a phone call on speaker, and then connect a bluetooth headset, but if I then hang up the call and make a new one, the iPhone shows me a list of audio sources (while automatically choosing "speaker").

I am asking for a friend who is paralyzed and cannot physically operate the iPhone without help, and is thus dependent on the iPhone automatically answering on speaker, but who would like to be able to make calls if he happens to be wearing a bluetooth headset without asking for help to either take the headset off and power it down, or to operate his iPhone by way of selecting the bluetooth headset as the audio source.

Thanks in advance