Secure SRST services cannot be enrolled while SRST is active. Therefore disable SRST with the no call-manager-fallback command.
Refer to Media and Signaling Authentication and Encryption Feature for Cisco IOS MGCP Gateways for a list of supported Cisco IP phones, routers, network modules, and codecs for secure SRST.
Refer to Cisco Unified SRST 4.0 Supported Firmware, Platforms, Memory, and Voice Products for the most up-to-date information about the maximum number of Cisco IP phones, the maximum number of directory numbers (DNs) or virtual voice ports, and the memory requirements for Cisco SRST.
http://www.cisco.com/en/US/products/sw/voicesw/ps2169/products_configuration_example09186a0080509462.shtml