I find it interesting, that I never had to set up NAT for either SIP or RTP ports.
Linksys PAP2T just worked, no NAT settings whatsoever, but it had a nasty habit of sometimes losing login to the provider until I simply changed port on the line that went down. Since then I tried setting up port forwarding to 5060-5070 and 16384-16482 hoping it would make the problem go away. That did not work, once in a few months one of the lines goes down and nothing but changing the SIP port# can fix it.