On Wednesday, 26 December 2018 10:59:09 UTC, Broadback wrote:
I have had a BT2700 and a Mybelle 360 connected successfully for some
time. Now I wish to have a Bluetooth enabled phone so have purchased a
Amplidect 400BT to replace the Mybelle. I seems they do not like each
other. when both are connected the BT 2700 refuses to answer calls, or
indeed ring.

Check the cord and socket wiring. If pairs are transposed or shorted, the Amplidect may be running on 2 wires, but the BT2700 need the third bell wire for ringing.

Try separate master sockets/ADSL filters on the BT2700 to recreate the 3rd wire ringing signal.

Owain