My elderly parents have a BT1000 cordless phone which is about 7 years old. In the last few months it has started 'beeping' at various times during the day. My Mum called BT and told her it was a 'scam' and that she had to block the last number that called her ... which was me!!! After managing to get myself 'unblocked', we changed the batteries, etc but can find no logical reason why the phone beeps. The display seems to show 'pasine' when it happens.
A loss of the phone line will cause most cordless phones to beep. Perhaps its "losline" or something similar that they are seeing?
It may be worth checking the lead to the phone to make sure its not damaged, and the plugs at both ends of the lead are not loose.
This is the help page https://www.bt.com/help/user-guides/phones/landline-phones/cordless-phones/bt1000
Is it a constant beep or single? As above, my 8500s will beep once if they think the line is disconnected, lost the connection to the base or when placed on the charger. All three of these can generate false beeps. One handset routinely reports that the line cord is disconnected. Another seems to have a poor connection to the charging dock, so that the slightest knock of the table it sits on causes it to momentarily lose the connection & it then beep as it's remade.