I assume that your phones are connected via a modem/router – have you tried restarting it and/or checking that the firmware is up to date?
The you could try disconnecting all the phones and BeoTalk, and then trying them one-by-one to see if CLIP works individually. Perhaps one of them is supressing the CLIP signal.
You could also try a reset of the BC6000 handset and/or its base: (make sure that you have backed up phone contacts if they are stored on the base):
– Reset base: Press (dot) 7 000 8
– Reset handset: Press (dot) 7 000 9
You could also try replacing the batteries in the BC2500s.