Asterisk Users kicking each other off?

I am having a bizarre problem with my Asterisk box (Debian: Asterisk PBX

Having read loads of howtos and tutorials I have managed to set the whole thing up as I’d like. I have about 10 users and also a Linksys SPA3102 ATA routing calls from my PSTN. I also have a Gradwell SIP trunk which also works fine.

I was a very happy bunny… until strange things started happening.

I began noticing problems when one of my users registered from his ADSL connection using Zoiper (freeware softphone). As soon as he registered, my SPA3102 changed it state to registration failed. Once he unregistered a few seconds later the SPA3102 came back online. The SPA3102 and my user had completely different usernames and secrets, so that was odd.

Then dialling a number from 1 user to another diverted the call to the wrong user with a totally different extension!!!??

Having finally got a fully working system - this problem is totally bizarre - and I don’t understand what could be going on… any suggestions?

Right - I’m finding out a bit more info here… if I set the SIP port on LAN clients to 5060 problems start occurring, if I set ports to 60,000 + it all seems to work. Can this be a port issues, surely multiple devices can connect from a LAN on the same external IP and port?

Have you got NAT in between?