Is it possible for one Cisco Callmananger cluster to be used for multiple customers with a handful of sites in an ISP type environment?
You can certainly segregate phone DNs, gateways etc using partitions and calling search spaces. The dialplan will be complex with such a setup, but definitely do-able. One thing to note is out of the box, the corporate directory is common to all phones. There are ways to use ldap scripts to solve this problem as well. (One caveat using custom ldap scripts for corporate directory is that the procedure is supported by Cisco Devcentral and not Cisco TAC).