If you ever find out what caused that random ringing please let me know
(email below is valid.)
My late f-in-l had an occasion a few years ago when his DECT phone
didn't clear down so the phone was engaged for almost a day - and as he
was in his late 80's it got us a bit worried!
As he had reasonable and reliable BT B/B I bought a Panasonic SIP desk
phone, set up a Sipgate account for him (no credit so he could just ring
us or our daughter who also had a Sipgate line), programmed the phone
and off we went.
After a year or so he told me he had turned it off as it kept ringing
and there was no-one there, and unfortunately mostly during the night. I
never did find out what was causing it, but I have the phone here and as
I have a line with credit from Sipgate I fancy having it sitting on the
end of my desk.