Currently having a strange phone problem for a week or so -

When the phone line and adsl are both in the filter, only the adsl works and the phone line has the engaged/busy tone.

When the phone line is plugged directly into the wall socket the phone works (adsl modem now disconnected)

Phone and adsl both in the filter - the landline has a busy tone again.

Have tried two adsl filters and get the same issue..

A chorus tech came to visit a couple of months ago due to a similar issue with the wiring, but where both the adsl and the phone line stopped working intermittently due to a 'line is busy' type fault.  As far as I know he just cut the cable to the extension lead and did not put in a master filter, as the wall socket is still the standard BT type.

Any ideas would be appreciated, I know it's house wiring related but don't want to pay for another Chorus call out if can help it..