Struggling with registration issue

This was the major issue that the client SIP Phone unable to register with the server intermittently. This is particular sensitive to the unifi original DIR-615 modem. From the server's messages log, you can see that the registration attempt was replied with "ChallengeSent" but some how, the client was not receiving this reply.

Trying out many methods including replacing the router with Asus 12HP and the latter seems to make the reoccurring lesser but still didn't fully resolve the problem. Even replacing the Asus router for TMS at Kota Damansara but still out of 6 ip phones (GXP1405), it's always one or two of them unable to register. It seems that the router was blocking the ip phone MAC addresses for some reasons.

The workaround is to replace those problematic ip phone and for Asus 12HP router, once it's connected, there is no issue so far for GSS amd TMS. But it is a hassle to replace the router as most customers prefer to retain the existing unifi router.

The worth to try possible solution was to set the outbound proxy server to the SIP server domain or IP address. So far, I have set this setting onto both TnL and LoanStreet (except the CS 2807 or 2804). Those ip phones still under close monitoring. If still not working, will have to figure out alternate solution

8.8.2014 - based on the last status, the registration fail doesn't happen to those phones with outbound proxy IP set. still under close monitoring