Fax tone on answering telephone

I have a problem which I just cannot seem to shift. Each time someone phones me a fax tone kicks in after the second ring. I have already stopped the fax service and have also disabled the service but to no avail. I have even tried changing the amount of rings to 15 before the fax kicks in but still to no avail. What do I do? I am demented with this. The only way I can speak on the telephone is to switch off my computer. I reinstalled Windows 2000 under the repair option just incase someof my configurations had gone amiss but it is still happening. A resolution to this problem would be very much appreciated.
Regards, Debbie