I dont think its the firewall.
The SIP service is on Sip Express. Thanx for testing it, I am in the same situation.
I used a free trial account from voipstunt.com and tested it, the voice was perfect, so firewall can be ruled out.
One observation from another friend, he said that the call was not following DTMF / RFC standards. I dont know how to check that, are you aware of this?
I used wireshark which provided me more evidence that is completely weird, It played back the captured stream. I was able to hear my voice on the number that I called (my mobile) which I never heard on Red5, there was no audio stream captured from my computer to the sip server.
I checked further and the error I found was “Destination Port Unreachable”, I dont know how to fix this and I dont know how it worked with voipstunt and not my SIP server. This is totally flipping my head .
I hope this works out for me, I would definatly post my ordeal so it can help others.