If your DSL cuts out whenever you receive a phone call, then that's indicative of a fault on your phone line. You may also have intermittent crackling on your line as another symptom.
Before going through the hassle of switching ISP, you should try getting the phone line fixed, because if that's the problem, it doesn't matter who you get your internet from, the problem will still be there.
As you are renting a phone line from BT, it would be quicker and altogether less painless if you could contact them directly to arrange an engineer visit. This will only be possible if you can complain about a problem on your voice calls though - if you tell them you're having internet problems, they won't want to know, and will tell you to contact your ISP.
As it happens I have exactly this problem and a BT tech is on his way up my driveway right now. I am also a VM ADSL customer.
---------- Post added at 10:57 ---------- Previous post was at 09:51 ----------
I have clarified the thread title a little.