[Serusers] serctl ping uri error 408

Hi, Just wondering if someone is facing similar problem. My setup is SER/MediaProxy/MYSQL on public IP, I have couple of dozen SPA-1001 all over the internet. The problem I'm facing is that many of these ATA SPA-1001 some times do not ring, so try to ping (serctl ping sip:550505050@myserver.com) and i get error 408. When i restart these ATA everthing starts working but couple of hours later, it's the same problem. Not to mention that (serctl showdb) shows that sip client is registered, however I still get 408. I tried the following: (but no success) 1. SPA-1001 change Register Expires: from 3600 to 600 2. re-INVITE TO 30 Can someone help, or maybe there are some manufacturing bug on some linksys SPA-1001, by the way all my ATA use uniform configs and firmware. If someone is willing to look into the ngrap, i can show it to you. Thanks, Ali...