OK, music on hold was not the issue - save that for another day.
We had what looks like a bug in SIP for one of the carriers. Caused a few issues with transferred calls. Worked around for now. We'll wait for them to agree they have a bug!
We had the recording system lock up - bug found and fixed.
But otherwise all was fine until 6pm. No stale calls. Everything working perfectly.
Then my Dad manages to make and retain 5 phantom SIP calls. How? FFS this is not possible. And no, he has no idea. Ho hum.