I think I know where your roommate is coming from.
For your part, I think the only thing you can do is get off the phone a.s.a.p, stating that you’ll leave a message for your roommate, and explaining that you’re busy. And rant here, if you have problems getting off the phone, and need to let off steam.
For your roommate’s part, it depends if it bothers her or not. That just may be the sort of relationship she has with her parents, and if so, it’s fine. If she wants to change it, it’s between her and her mother.
I didn’t speak much to my mother from about the age of 12. We conversed the bare minimum to keep things moving (I’d ask for money for a school uniform, go get my uniform, or books, return the change with the receipt, and that was that). I moved out of home as soon as I turned 18 and finished high school. I lived with Mr Goo (not married at that point) for two years, and supported myself through uni. During that two years, I probably visited my parents maybe six or seven times. We deferred uni, and went overseas, backpacking. Mr Goo encouraged me to try one last time (it was the final time, if the response wasn’t good, I had decided to cut my mother out of my life forever.) and see if anything could be resolved. We ended up talking quite well, and I was thinking that there may actually be hope for some sort of relationship there. (this is at the age of 20, after barely speaking since 12) Anyway, to cut a long story short, my mum gave me a phone card (the bills were to be sent to her) and asked me to call occasionally and let her know how we were. I called from Indonesia, after a month of travelling, and then from Manila, after another month. The call from Manila included the information that we were heading south to an island, and taking some scuba lessons. The island we got to was stunning, we ended up doing several courses, and stayed 6 weeks (we had to extend our visa) in this beautiful place, which had no phone lines. My mum went apeshit, calling Mr Goo’s mum, and the embassy daily.
I wrote that whole big scenario because it kinda shows that even after 8 years of no friendship or displays of love, two of which were totally self-supporting, mothers can still be insanely protective and react to things that others find ludicrous. I think some chemical is released when parents have kids that means they can’t ever be laid-back about their own children no matter how old or responsible they get.
If this bothers your roommate she needs to sort it out. I told my mother that we were holidaying, with no set itinerary, and not to expect calls at all, let alone regularly. Quite often we were in regions where there were no phones, or our plans changed and I made a point of never phoning the same time (like never two saturdays in a row) so it wouldn’t be expected. For a laugh, a year later, when we were going on our honeymoon, I had to tell my mum that the island we’d chosen had no phone lines, but if it did we still wouldn’t call, because it was our honeymoon ! What was even funnier, was four months later, we visited New Zealand and mum didn’t even bring up the whole phone business (knowing we don’t like being restricted to set plans) even though she knew I was planning on doing stuff which freaked her out, like sky-diving, bungy jumping and snowboarding. I think she’s finally trained ! To make the point, I made sure I called several times, after the first three days. She didn’t even expect a call when the WTC thing happened, because (and I quote) “I [mum] figured you’d be off having too much fun, and wouldn’t even know about it”. Mother’s behaviours can be trained, it just takes persistence, and you have to remember that even though she isn’t calling, she’s thinking about it, and it’s only the constant training that’s stopping her… she’s itching to call and check that you’re o.k, just restraining herself. Maybe your roommate would enjoy my story, since you don’t have problems with your parents I have no idea why I wrote all that… Oh, well, share it with your roommate. 
Anyway, about your roommate, AotL, not much you can do but rant here and try not to get involved. It’s pretty much between your roommate and her mum (though it sounds like you know this) and you need to figure out some ways to extract yourself from the phone when her mother calls. I know how hard it is to get off the phone without coming across as rude, when the other preson doesn’t want to let you go.