This sounds like waaaaaaaaaaaaay more trouble than it's worth. Seriously... Why try to date someone whose mom will constantly get in the way? You'll probably spend more time sneaking around than actually hanging out with her. There are enough girls out there that you shouldn't bother with this one.
The best way to deal with this is to make her do ALL the work. If she really wants to date you, she can figure out a way for you to get together and avoid her mom. Her mom, her problem, she deals with it. If she can't, it's probably not worth it.
Another suggestion: why not talk to her on IM? The girl I'm currently dating I've set up nearly all the dates via AIM... I think I've called her once in the last 2 months.
I like instant messenger because I don't have the attention span for phone conversations, so I can do other things while I'm chatting. I think my current circumstances are special in this regard, and I don't recommend AIM in most cases... But when you can't talk on the phone or don't feel like it, it's pretty nice.
A few random responses:
1. 15 year olds don't know sh!t. When I was 15, I thought I was smart too... But I wasn't. I'm almost 18 now, and I still don't know sh!t... But I know a lot more than when I was 15. With age comes experience. Respect that.
2. Long phone conversations are bad. I know this both from theory and personal experience... Very rarely will anything good come of talking on the phone for a long time, except in a few very specific circumstances, which are so rare I can't even think of them right now.