This issue is documented in Cisco bug ID CSCin53117. It appears that in Cisco routers 26xx (2650/2620/2691) and 3640 routers, voice interface cards option AIM-VOICE-30 is missing.
Users find AIM_VOICE_30 missing in slot 0 for an existing gateway. When you go to the gateway configuration page, these slots show as not configured. Once the user presses update, the previously configured end points for slot 0 are deleted. Users need to select AIM_VOICE_30_SLOT_0 as the module, and re-configure all end points under slot 0.
The above issue can solved with an upgrade to Cisco CallManager 3.3(3) SR2, which is available on CCO. The Upgrading to Cisco CallManager 3.3(3) documentation assists you with how to upgrade the Cisco CallManager.