IN F/W 3.3.3_232201, I added a SIP account, and left the expiry time at 10mins.
I noticed the repeat REGISTER message in the logfile every 10mins, os I extended it to 1Hr (3600mins).
However, it is still (according to the logfile) renewing the registration every 10mins....
Another account I added (with a 1Hr refresh time) IS renewing every 1Hr (more-or-less!)...