Interesting problem. I am running CME 4.2 on a 2801 router. I have a 7960 phone that can register no problem to the 2801. I have an FXO port that connects the router to the PSTN. I am also using IOS Zone Based Firewall for the firewall.
When I try to place a call, it goes through however I get one way audio. The PSTN end can hear me, however I cannot hear them. If I disable zone security (remove zone membership) on that interface it works fine.
The thing is, what zone pair would I have to define to enable this to work? It should just be a self zone which is fully enabled by default.