Sorry, should have stated this in the first place.
They are h323 1760 routers which some are connected to ISDN2 circuits and others connected to ISDN30 circuits.
We also have 3 x 2800 series routers in London configured as MGCP gateways.
The 1760 routers need SRST capability which our current IOS (12.4T9) supports. We currently run CUCM 6.1(4a) and have had no problems with our voice gateways so far. Was hoping an IOS upgrade to the latest 12.4T range would suffice when we upgrade our CUCM cluster to version 7.1(3).