Michael, 

Here are the differences between the systems that I determined from the two SIP 
traces: 

* Working system: no NAT, phone codec: G.729, Asterisk codec: G.729 
* Non-working system: NAT, phone codec: G.729, Asterisk codec: A-law 

Does the conversation have two-way audio prior to the hold? If it doesn't, your 
problem may be caused by NAT and/or codec transcoding. NAT is notorious for 
causing one-way audio and transcoding G.729 requires a commercial license from 
Digium. 

After those two possible causes are ruled out, the only other thing that I can 
think of is a missing format translation path for the music-on-hold files. Does 
the AsteriskNOW system have modules loaded for both formats? I don't use either 
format, but I believe the required modules are "format_g729.so" and 
"format_pcm.so". 

Regards, 

Matthew Roth 
InterMedia Marketing Solutions 
Software Engineer and Systems Developer 
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