"Slamming" is illegal over here, and it was a common practice among the phone companies. They took any kind of contact as a willingness on your part to have your phone service switched. ATT did it to me one time, taking my long distance service from MCI. I called and harangued them, and of course they played dumb, but my service was switched back the next month, and I think I reported it to the state attorney general's office, or whoever was taking those reports at the time. They still try to do it, but people are pretty wise to it now.
As a thought--phone wire is cheap. It might be worth your effort to change out the line from the wall to the computer just to see if that makes a difference. You can buy pre-cut pieces of phone cable here with the ends in place for a few dollars at the hardware store, the electronics store, even the grocery store. When problem solving, always try the cheap fixes first. It saves so much grief later. Does your house line run in the attic or basement where you can get a look at it? A visit to that line might also be fruitful.