I had a set up where incoming calls and outbound calls had audio but when a call was redirected without being picked up to an external number there was no audio. This is probably because direct media is not working through NAT. To work around this I did something like this:
The directmediadeny will make sure direct media will not be used for the IP of the external SIP server. Most of the times this is a quick fix, direct media should work if your NAT set up is OK.