1. Always maintain a calm, collected tone
Keep your voice even, keep your tone low, and do not lose your temper. If you find yourself getting upset, place the person on hold for a moment, take a slow, deep breath, and pick the phone back up. If they have hung up on you, mention it when you call back.
2 . Before you call, outline the situation for yourself, and decide how you want it solved
Write down several options you would be willing to accept, and keep the page in front of you when you call.