1. Is there a problem with one of your Filters as if it's not working then when someone rings it upsets the BB connection.
2. Seem to remember that call waiting can effect BB as well.
3. Normally you should have only 3 or 4 phones physically connected to you phone line as each phone = 1 REN and you can have up to 4 REN per line.
4 Typing Broadband or call waiting in the search area brings up some old posting that people have solved their BB problems, try that