Ah, so that is why PAP2T did not need any port forwarding!
Fascinating.
On the contrary, Grandstream HT702 simply refused to work if I did not forward all ports it requried. And the funnyest part was that when I repaired PAP2T and put it back in service, ports 5060 and 5061 were still being forwarded to the HT702’s IP, but PAP2T still worked on a different IP with those same ports. This device just seems to work no matter what.
Of course now I removed all port forwarding entries for VOIP from NAT, like it used to be back when I was using PAP2T.
How did your PAP die? It may be repairable. Mine went into a cyclic reboot after I shut it down for half an hour to test HT702. Ended up re-soldering one of the power transistors and ceramic capacitors on the phone lines side after finding with a oscope that there was no PWM on its inductor. It works fine again.