Thanks. Not certain that is the cause though.
Both GSC3570 units do have the latest firmware installed (done when initially installed). Son is certain he did factory reset the kitchen unit after updating the firmware, The office unit was installed a few weeks later and he ‘thinks’ he would also have reset that after applying the latest firmware. Anyway, as a precaution, he did another factory reset today so we will see if that does make a difference.
However, we also made a test call from the office to the kitchen unit (kitchen unit has not been re-booted for some time - possibly more than a month ago). Despite the fact that he is certain he did factory reset it, there was no ringtone on the kitchen unit. The LED at the back of the unit was flashing GREEN, and I was able to answer the call OK, just no ringtone. I was standing straight in front of the unit waiting for the call. Tested it twice, same result.
Also, now not so certain about the time it takes for the ringtone to stop working. Yesterday I was thinking it was about 2 weeks since it had happened but my son told me today that he had re-booted the unit 3 or 4 days ago as a precaution. Both units have now been re-booted today so we will keep an eye out to see how long before the ringtone stops working.
Another bit of info from my son today. The last time the ringtone stopped working on the office unit (~couple of weeks ago) he went into Settings > Basic > Sound, and selected a different Ringtone to see if that would fix the problem. However, there was no sound when he selected alternate ringtones (normally there is so you can test/hear the ringtone before deciding which one to select). So the ringtone would not work even from the Settings menu.
I just checked the call history. Last unanswered calls were 4 Apr (from the GDS3710 gate station) & 29 Mar (from the kitchen GSC3570 unit). In the former case (Sun 4 Apr) it is possible I was not in the office at the time, so I cannot be certain there was no ringtone that time. However, that (4 Apr) also shows as a missed call on the kitchen GSC3570 unit.
Possibly a memory leak issue?