loseing broadband connection

hi i am wondering if anyone could help with this ,

i have got broadband with tiscali the problem is that when i make a phone call i can connect to the internet but when i stop useing the phone the internet connection drops and the computor says no dial tone ,
we have changed the phone , the modem , the filters, the problem is still there we have talked to tiscali, and tiscali says that the internet connection is ok , bt says there is no fault on the line
what else could it be ?

This sounds very much like a high resistance line fault. You must get in touch with Tiscali and insist that they escalate the fault to BT.

- Easier said than done with Tiscali !

That's why I used the word "insist" rather than "ask"

the problem was not the phone line , telephone , socket , it was the filter allthough the filter was tested on another computor and tested ok and we tested a new filter which did not work so we bought a different filter and it worked ok
has anyone heard of this before ?

many thanks for your help