I have a BT land line socket box located inside my hall. Through a hole in the interior wall an ordinary BT 5m cable that has four brass pins within the different connectors at each end goes from the BT box through this hole to my BT DECOR 1200 phone. It has a dialling tone and works fine. And so does a new BT DECOR 2500 I recently bought.
I decided to put the BT DECOR 1200 land line phone in the hall and used a double adaptor one for the next door phone and one for the hall phone. The BT DECOR 1200 did not have a dialling tone and obviously doesn't work. But the BT DECOR 2500 did still work in the room next door.
I tried using the BT DECOR 2500 in the hall and used another BT cable with four pins from the hall BT socket, there is no dialling tone. It is the same with a BT cable with only two pins either end. Both are new cables. And I have used a multimeter to confirm that there is connection between the connectors at either end.
It seems that both my BT phones are wedded to this particular cable.
I repeat both phones work in the next room but neither work in the hall. How can this be. The only thing of consequence was a terrific lightning storm recently which caused land line internal socket boxes nearby to be replaced by BT engineers. Our land line apparently survived but with the problem described above.