Taily, try a different filter. Actually, if that doesnt work, you might want to change all the filters (perhaps borrow some known good ones?). Obvious thing I suppose is to plug in a normal phone into the socket usually occupied by the fax and see if that works OK. If it does, the deep lake option may be the wisest.
(... resists temptation to ask if the peeps at the other end are on the phone... 