In all my years using a moblile phone I have never heard this before. About 3 minutes into a conversation and I hear what I can only describe as the 'Payphone pips' that you used to get when you were running out of credit. Then about 10 seconds later the call terminated with the usual call fail tones.
And no, I hadn't run out of minutes - Over 490 left this month!
The only thing I can think of is that it was some sort of basestation pre-emption (i.e. being chucked off for higher priority traffic). Does anyone know if this is what 'payphone pips' mean on a mobile network nowadays?
Pips were only audible to me, not the person I had called.
I've had callers having difficulty in getting through to me.
It's a hilly area, with difficult reception, for themselves, but not at my end.
Common explanation given is that the connection to the network is about to drop, or be forced to drop, as you suggest.