SMS works on xTelewest.
Yes, cable in most areas use a different CLID system.
The 'BT' version sends the CLID before the ring, so SMS phones know not to ring the phone when it 'sees' the SMS service centre phone number.
The cable version sends it after the first ring, so unless the phone has been designed to ignore the first ring of any call, the phone will ring and then when it sees the SMS service centre phone number the SMS section takes over.
The 'BT' version of CLID also has CWID - Call Waiting Caller ID - So if you have call waiting and someone calls you can see whilst still on the call who the waiting caller is.
The cable version is a version of BellCore. The BT version is ETSI FSK (UK) with PR.
www.bt.com/bttext
Try a send centre no: 1470P17094009
May need a couple or three P (pauses) on cable as the cable system needs pauses (the BT system can get away without them).
Receive centre no of either:-
0800587529
0800587529* - replace * with a digital 1 to 9
08005875290