Joy reports this occasionally when I ring. I have a DECT phone. They had water in the wires.

There is a condition with UK phones where if there is water in the wires and it bridges any bare wire you can get the whole of the PCM breaking in - probably sub-harmonics of the megaHertz frequencies.

I heard it on a phoneline that died a few days before news of my sister's death. They were ringing from NZ and they could hear the ring tone (used to mean the phone was there, but not now), which they assumed was "no answer". Bro-in-Law had to get the police to call round. I was at a Festival, so SO (at the time) had drive the 100 miles to tell me. And it got more complex after that.

My reaction would be to get the phone to do it inside of a minute then immediately call the engineers and see if it does it. At least they will hear the problem, hopefully.