I don't know if this works yet, but it hasn't beeped for 10 mins now... so fingers crossed.
I changed the battery on the handset of the phone that was beeping, but it kept on doing it. Then for a reason unknown to me, I pressed the BELL icon on the bottom left of the phone the star (*) button and clr button and it hasn't done it since... yet.
As I said, fingers crossed. My phone is a BT Freestyle 25000 2 x cordless phone.