Your 30 second registration interval is probably being enforced by your SBC. If you look at the Contact header in the registration response from your SBC to your phone you will probably see 'expires=30'.
You can check the values of the 'Reg Min Expires' and 'Reg Max Expires' on the SIP tab in the Administration console to make sure you're not restricting it there as well (I believe the defaults are minimum of 1 and maximum of 7200). You'll need to make sure you're logged in as Admin and using the Advanced view to see those parameters.
If you'd like to paste a registration capture to this post I'll take a look.
-Adam