Try time of day routing ---https://supportforums.cisco.com/document/54501/how-configure-time-day-routing-communications-manager
or another way to do it is use a time based access-list on the locally connected router so the phone subnet can only speak to the call manager at that time
http://www.cisco.com/c/en/us/support/docs/security/ios-firewall/23602-confaccesslists.html#timebasedtimerange
