My daughter had a similar problem as you describe, also that when anyone rang the broadband would cut off if she answered it.
I tried all sorts of things, changed the filters, got a new adsl modem from the ISP, reinstalled ms networking and client for ms networks, etc.
Then as a last resort I changed her portable telephone to a BT one that I had spare. Since doing this it would appear that the problem has gone away. I could not find any reference to it or any other similar problems people had, but I can only assume it was something to do with the digital signal frequency of the old portable phone.
Worth a try, borrow someones