I have a problem that is almost the same as yours, but my case is inverted. In my case, when I set behind_nat to true, the SIP register intervals are as expected. But if I set behind_nat to false (which is the default), the SIP register intervals are way too high, something like 2000 - 3000 seconds.
Have you ever found out what is the underlying cause of this? Also, I read your post regarding the SIP options after a register (with the Max-Forwards header set to 0): SIP OPTIONS after call - #4 by jmorrison876. I have found that this also happens in my case. I was wondering whether you discovered something in this matter. It might correlate to the SIP (re)registration issue.