I don't troubleshoot the firewall side of SIP every week, but from what I can tell our SIP NAT ALG has been stable for a while. I very rarely see issues with our SIP NAT ALG ( although I don't work on the firewall team), and I would put my trust in it. Even more so if you're going through an ATA. After 12.3 mainline or so, it hasn't given me any problems.

Nicmath, thanks for your input.

However as far as i know SIP ALG is a new feature to IOS, only available from 12.4(15)T onward. I might be wrong since I only started looking into SIP trunk very recently.

Thanks again.

This document may be helpful for you:

http://www.cisco.com/en/US/docs/ios/12_2t/12_2t8/feature/guide/ftnatsip.html

This feature has been around since 12.2(8)T it appears, which has become quite a mature feature.

I can assure if the feature had just been implemented in 12.4(15)T my opinion would be different.