Thanks, but I've now done some significant trouble shooting and am pretty sure that it's either a part going wonky or (I pray) a software issue.
Apple Ear Buds, both brand new, one pair came with the phone. Doesn't always happen but once it does doesn't stop until you hang up and restart the call, then it's fine (without, per experiment 53 today, touching the earbuds) Today, one of the times, I was sitting on my couch with the phone utterly still on the couch next to me when it started. Not mechanical.
Can you tell I spend a lot of time on the phone? 🙂
Two phones, same issue.
Need this admitted to and fixed. Where is Woz when you need him? (speaking as a Prius owner!)