Fax on my phone line ??

[Piet Bloubul]

I had broadband installed a while ago and it work fine but when i receive a tel call the line sound the same as a fax line. As soon as i disconnect from the int the line is clear again.

This don't happen everytime, but with about 80% of the calls.

I phoned my isp and bt, both say their sides are clear. I don't have a fax on the line, but did use the pc fax, but i've disabled and now uninstalled it.

What can cause this ?

Any help please.