Consider standing up when you are making calls. If you are reluctant to speak on the phone, your voice may sound weak, and others may sense this and intimidate you. When you are standing your voice will be louder and more assertive.
Consider a script for each phone call. If you know what you want to say, you can often barrel through a conversation.
The headset's a good idea.
Any trick that can make you feel as if you are in control of the situation will help you with this.