Rob - Here's another thread that provides just a little more info about the 79xx SSL support for UC500.https://supportforums.cisco.com/message/3133332#3133332
Most likely the phone has and older firmware load already installed. 79xx firmware cannot be directly upgraded from 8.3.3 or earlier to 8.5.3 or higher. http://www.cisco.com/en/US/docs/voice_ip_comm/cuipph/firmware/8_5_3/english/release/notes/7900_85...
The default keepalive setting for ephones and telephony-service is 30 secs. If configured in ephone config mode, this keepalive setting takes priority over the global telephony-service keepalive.From the CME command reference http://www.cisco.com/en/...
