I think what should have been asked was if you used the same power supply for the three DECT units, or the power supply that came with each. If you just used the one then that could be the problem.
Also is the DECT base connected at the master or at an extension?
to re-iterate - only one DECT base is used - I have three available so I can test the issue.
The problem exists with all three DECT bases.
The power supply used is the one supplied with each DECT base
The Master socket and the test socket behind it have been used as well as various sockets in the house.
All exhibit the same issue no matter what.
Two of the DECT bases are BT brand - 8500 and 3760 the third is a Response technology company item.