To update this post, I was in work Wednesday/Thursday this week. Wednesday morning there was a certain amount of stress from the lovely Jacqui, trying to get the phones working and not knowing why they weren't on. The last time I'd been aware of the phone system in the building was when the digital exchange was in a beige box the size of a chest freezer up in the coms room, but when we went in there was only empty space where it used to stand.
After a bit I managed to trace the phone line to some Samsung rack units of about 1/50th the size of the previous unit, that were not showing any signs of being powered up, and from there became aware of a faint high pitched whine over the sound of the other kit that appeared to be coming from a UPS. Hum. The UPS unit was, in fact, very interrupted, and about 10min after plugging the phone control system back into the mains communication was restored.
Gotta love coms room wiring :-)