There's really not a lot to screw up in that though.
External IP: my static IP
Local Networks: 192.168.1.0/24, 192.168.2.0/24, 192.168.101.0/24, 192.168.102.0/24 (the one actually involved here is the 192.168.101.0 scope)
Port Ranges (49152-49999) - but this should only be used when going outside the firewall, which a voicemail call would not.
I've tried two STUN servers, and when they're bad/empty, I can't even get the call to ring in (for obvious reasons).
And I still come back to, a codec change wouldn't affect NAT, if it were in play.