I originally posted this on UK.telecom but have had no response so far so thought I would try here.
I have a BT versatility at home which has been working fine for 5 years or more. It has x2 black analogue modules, x1 blue answerphone module and an 8 extension expansion module.
The problem is that extensions 26 and 27 cannot be rung. From a POTS internal phone you get the steady number unavailable tone. From a systemphone the name of of called extension briefly shows then the message "Extension has gone". I cannot find that message documented. The same happens whether the extension phones are plugged in or not and even if the wiring is removed from the connector on the main unit. Normal behaviour with empty terminals is that the extension appears to ring from the calling party's perspective. So I think it is something in the main unit.
It has been warm and cold booted but problem still present.
You can still ring out from extensions 26 and 27 to internal or external numbers, even though you cannot ring into them.
When power is cut to the switch, then in fallback mode external line 1 is meant to be connected to extension 26. That still works.
When you make ext 26 or 27 part of a ring group then they cause ringing of that group to fail.
I have had a look and cannot see any bits of wire shorting out any terminals or pcb tracks, but might have missed something.
The only significant event I can think of that might be relevant is a thunderstorm about a month ago. I suspect the problem might date from then as I did have to reboot the switch at the time, but have only just noticed this problem with these 2 extensions.
Anyone any ideas?