Check the Compatibility Matrix
Cisco posts a compatibility matrix on Cisco.com (see Figure 1-2) that helps you determine whether the parts of the IP telephony system you want to deploy are compatible with other parts. You need to be certain that all upgrades, firmware loads, applications, gateways, and other hardware devices are compatible with the version of CallManager you plan to deploy. Access the compatibility matrix at the following link or search Cisco.com for "CallManager compatibility matrix" (when you get the search results, look for the Cisco recommendation to the right of the results screen):
http://www.cisco.com/univercd/cc/td/doc/product/voice/c_callmg/ccmcomp.htm
Figure 1-2 CallManager Compatibility Matrix on Cisco.com