Q: When I talking on the phone my phone wants to switch to FaceTime mid conversation.  How can I avoid that happening?

Remove whatever case and/or screen protector you have installed on the phone. Some of them can interfere with the proximity sensor. Don't try to cradle the phone between your head and shoulder to free up your hand. It tends to end up at an awkward angle that doesn't allow the proximity sensor to register that it's up to your ear.