I've not had exactly the same problem as you describe, but similar problems in the past for me have all been solved with a new filter....probably the easiest bit to replace too.
Have you done any other troubleshooting? Start off from the point where you haven't got anything at all plugged in to your phone socket, and gradually add bits until you get a problem (apologies if you've already done this).
Edited to add that the last time I disconnected/re-connected our wireless phone from/to the filter it didn't work - I had to force it to talk to the base station by pressing the "intercom" button for some reason. Once I had done this, no problem.