Hello go.stu,
We understand you are trying to adjust your iPhone settings while you are listening to Maps directions and on a phone call. It seems you may want the Maps spoken directions to take priority when you have a phone call. You mention you'd like them to come through over "the speaker from maps rather than a prompt sound" on your iPhone speakers.
- Are you using CarPlay with your vehicle?
- Is your iPhone connected via Bluetooth to your vehicle otherwise?
- Are you no longer hearing Maps directions when a call comes through?
You might check the settings within Settings > Maps > Spoken Directions to enable "Directions Pause Spoken Audio" and see if that accomplishes what you'd like.
Change settings for spoken directions in Maps on iPhone
Reply back with those details if that setting doesn't help and we'll provide additional guidance.
Take care.