I am assuming that you are using your phones on the standard analogue PSTN (Public switched telephone network) and not via a converter/router ? Please confirm..
Assuming that you are using your Beocom's on a standard analogue phone line, then it is unlikely to be the phones themselves at fault. As the phones react to the "Ring Indicator" signal from the exchange. However, it could be caused by one of the following:-
- A fault in the phone exchange.
- If the line is also used for broadband (ADSL) then one of the microfilters could be faulty.
- If the line is also used by broadband, it could also be caused by the router.
I suggest you connect just one phone on the first phone point connected to the incoming phone line and leave everything else disconnected and test it. If you still have the problem, then your problem is almost certainly going to be a fault in the public phone exchange and you will need to call teh phone company.