Phone A:
Settings > audio > speaker
Click the triangle button to have audio speaker activated
Phone B:
Settings > audio > speaker
Push to talk and speak
When phone B is talking, then a lot of feedback occurs in Phone A. Presumably phone A's echo cancellation is not working and the sounds coming from it's speaker are being picked up by the mic and causing feedback.
Phone A: Settings > audio > speaker Click the triangle button to have audio speaker activated
Phone B: Settings > audio > speaker Push to talk and speak
When phone B is talking, then a lot of feedback occurs in Phone A. Presumably phone A's echo cancellation is not working and the sounds coming from it's speaker are being picked up by the mic and causing feedback.
IOS 9.1. iPhone 6.