[Users] NAT ping best methods

Hi, I am using Openser 1.2 and all the UAs that register to it are only PAP2 or SPAs. But all these UAs are on a private network. I tried using the NAT ping method from Openser to keep the NAT port open, but still sometimes the NAT port gets blocked by some NAT boxes. I tried keeping the interval as 25 seconds. So then I tried sending Keep Alive Notify messages from the UAs itself towards my Openser, but there I see a strange problem. The problem is like the device does not send the notify message from the same port which it registered from in the first place. So the port which is stored in the locaion table gets blocked and again the user is not able to receive calls. The user is able to receive calls only after it registers again but then again it is inconsistent. Has anyone worked with PAP2 and Openser and has got some best method to maintain the NAT ports open so that the user receive calls consistently. I also wanted to know what is more efficient, the UAs sending keep-alives or Openser sending NAT-ping? Thanks in advance, w/regards, Jayesh Add online presence to your web pages, find out how here - http://messenger.yahoo.com/addpresence.php