I just finished diagnosing some severe problems with my Elastix configuration and though I would share the solution. I explored many blind paths, but ultimately found that the latest firware release for my Netgear router was to blame. Specifically, the (otherwise reliable) Netgear FVX538 running firmware 3.0.5-24 (the latest) has a bug in the way it handles SIP. A SIP ALG feature was introduced with this firmware, but there is no way to disable it. The only fix it to downgrade the firmware to 3.0.4-19. Alternatively, you can beg Netgear support for a beta version of 3.0.5 which has SIP ALG disabled. Here are the specific symptoms I experienced, there may be more. - Using the Voice Pulse ITSP, inbound calls work fine, but outbound calls will fail. - A Linksys PAP2T located on a VPN will register successfully, but after some indeterminate period of time (measured in hours) ultimately fail to re-register. - When successfully registered, outbound calls to the PAP2T will fail, while inbound calls work fine. - Oddly, a D-Link ATA (locked to Vonage) works fine. Downgrading to 3.0.4-19 cleared up all the problems.