Problems with DECT cordless phones
If you subscribe to BT's Caller Display service some DECT cordless phones will behave differently when connected to BT TwinTalk. Some DECT cordless phones will not display the number of the Caller unless the TwinTalk to which they are connected allows the phone to ring. For a list of phones which are are known to work in this way see here.
Some DECT cordless phones produce a short burst of ringing in response to the Caller Display Signal on your line. This signal is sent to your line, to identify your caller, and it is sent before ringing is applied to your line. This signal may cause your DECT cordless phone to ring once when either of your numbers is called even when the TwinTalk to which it is connected has been set to block calls for the number that has been dialled. For a list of phones which are known to work in this way see here.
You can prevent this from happening by following these steps