asterisk users - Jan 2014 - Cisco 7940 SIP 8.12 no audio when using Outbound Proxy

Hi All,

Simple scenario:

7940 SIP><NAT Router><INTERNET><Asterisk SIP B2BUA w/Public IP
Inbound/outbound calls work fine 2 way audio, features ok, no issues
that I can tell so far.

7940 SIP Using Outbound SIP Proxy><NAT
Router><INTERNET><Asterisk SIP
w/Public IP
Phone registers, call in/out SIP Signaling traversing the proxy ok no
audio on phone, SDP messaging is correct.  Not using media proxy,
media flows between Asterisk and NAT router to phone, no return media
from phone to Asterisk.  This only occurs on inbound calls to the
phone, when the phone makes outbound calls, audio sets up fine 2-way.
On inbound call to the phone, I can see media going to the phone but
don't hear any on the speaker/handset, no media flowing out of the
phone back towards Asterisk so no audio on that end either.

I'm testing various phones, the Polycom and Cisco SPA5xx lines work
great using outbound proxy.  So I'm certain this is a Cisco 7940
problem not accepting RTP due to some internal security check with SIP
signaling and media coming from different ip addresses or something
like that.

So testing a bit more, I put the Cisco 7940 on a Public IP, seems to
work fine, audio sets up 2-way inbound and outbound calls.  So now I'm
thinking it is a NAT issue, but only when using outbound proxy,
doesn't make sense, now I'm really confused.

Any feedback is appreciated.

Thanks.

JR
-- 
JR Richardson
Engineering for the Masses