There is something not completely clear when you use Linksys routers.
Maybe there are others out there, but the linksys “remember” the internal I.P. address from where it got the last H323 packets.
That makes some I.P. phones and others like ATA ‘s, work behing the router without any port forwarding or DMZ zone.
The router keep sending the packets to the internal I.P. where they got originated the first time, even if you change the internal I.P. of the device.
That forces you to reset the router when you change the internal I.P. of the phone or device.