Actually a subject.
Why might that be? the phone is stupid in all senses(on 1 external IP, you can hang just one phone), model 1603, as described in the instructions on ports 1719 and 1720 — they are forwarding does not work — NAPT error
Why might that be? Plugging in the router and the port forwarding is incorrect or makes a phone with portforwarding does not work, only with DMZ? or maybe port some besides used which is not reported? in this case, how does it know?
DMZ is not suitable due to the fact that lots of other service ports is also trying.