lincensing wasn't a issue for call manager, until I believe 3.3 and up, were there will be licensed number of users. check the release notes for the version you are using.
ICS7750 is rated for 500 IP phones - there are a few issues with that however; depending on what is running (services, applications, etc). Must remember that the SSP is a 2900 switch - which is the backbone of the 7750. We have a 7750 running with 2 CCM, 1 Unity (SPE310's) and 3 MRP300 (two PRI's and 1 T1 to a Fax Server) - There are about 400 users on this system - this unit is pretty well max'd.
hope this helps - lots of info on CCO
