Well I'm certainly no expert on this stuff, but maybe you should start responding to Mr. A like you would if a friend called. If it's convenient to talk to him when he calls then pick up the phone and say hello. If it's not convenient then let it go to voicemail and call him back later. When you talk to him if you don't like the way the conversation is going then cut the call short either by telling him why or simply that you have "things" to do - your choice.
