I assume you are running the 7.5 firmware which is the latest. Here is a link to that version's compliance with RFC 3261 which shows that it will respond to OPTIONS only. I don't see mention of keepalives in the entire guide, which leads me to believe that it will not send those either.
Cisco SIP IP Administrator Guide, Version 7.5 - Compliance with RFC 3261
You may be able to change the REGISTER timer on the CLI of the phone:
# Phone Registration Expiration [1-3932100 sec] (Default - 3600)
timer_register_expires: 3600
You posted this question in another message board where you said the phone was in a non-CUCM environment. What do you have the phone registered to?
