Dad gets angry if mom uses the telephone

My dad is diagnosed with Alzheimers and has turned from being a kind man to someone who angers extremely easily. He never yelled, never swore and always wanted my mother to have a relationship and for me to even call my mom more not less. Now as his Alzheimers worsens he gets angry if she takes a phone call from me. I live in AZ and she's still in WI so I cant just stop by. She just lost her father and also needs support. Its gotten so bad she is waiting until my dad falls asleep but then has to hang up before he sees. Has anyone else dealt with this and what did you do?