Talking on the phone is something that's really difficult for a LOT of people--even beyond people with AS. For people who stutter, talking on the phone can be entirely unbearable. It's hard because all the visual cues you might use to learn what the person really means are totally stripped away, and all you get is their voice--and the person on the other end may use a prosody (speech melody) that's difficult to interpret.
Unfortunately, the only way to fix it is to practice. I still use scripts to help me with phone calls. I used to have to write everything out, but now I can just write a few phrases for my key points. Maybe the best way to practice IS with someone you know really well--like you're mom. Even if you don't really call, and just use 2 different phones in your house. Let her know the topic, write out a few things to say, go in another room, and have a short conversation. Try with a sibling, or an understanding peer.
While on the phone, it helps to pause and take a deep breath whenever you start to get nervous. Keep telling yourself that it's really okay. After all, if something goes wrong in the conversation, what could happen? Think all the way through that question, but don't let your imagination run too wild. The consequences, when you think about them, aren't all that bad. When you look at it, the results that you are afraid of (messing up, having the other person thing less of you) are actually not all that consequential--most people will forget if you've messed up, and if the person thinks less of you, well, that's more of a reflection of them being a jerk.
Was that helpful at all?