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.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.