Sounds like your phon eline has problems.
Change your ADSL filter, make sure every socket used has a filter on it, Fax, Phone, Modem, all need to use Filters on their sockets.
If still not good after that, when your line is playing up, call your phone line or ISP and ask them to run a qulaity check on the line to checj Noise and tolerance.
Your line playing up causes such issues.
Hope this helps.