Maybe the PBX lost its clock? Sure it did. It has no RTC, or at least no battery backup.

Just a little update from my future self:

I rescued the fully-featured variant of my ISDN PBX from the garbage bin. This one has all the bells and whistles, supports ten instead of just six analog phones, supports a module to add a second external ISDN line and a second internal ISDN bus, supports a second door line module. But it is basically the same hardware as my PBX. My PBX just has a few components and connectors not mounted. I opened the rescued PBX and had a look around, and found "the usual suspects". And in a corner of the PCB, I found pads for a CR2032 battery holder and an eight pin RTC chip next to it.

So the hardware seems to support a hardware RTC including a battery backup. But at some time late in development, the manufacturer decided not to mount the RTC components and instead rely solely on the software and ISDN network features. That's more than just annoying!

On the other hand, the RTC battery would have been placed in an "unreachable" area of the case, locked behind sealed screws. So after about five to ten years, the battery would be drained and the PBX would either refuse to work or fall back to the current behaviour. To fix that, the manufacturer would have to re-layout the PCB to move the battery to the connectors area, or add another removeable cover to the case to allow access to the battery holder.

Removing the RTC and battery from the design allowed the PBX to work continously for more than twenty years. So after all, getting rid of the RTC and battery was probably a good idea.

Alexander