MetaSwitch/Acme multi-line hunt group audio delay

We interface between Polycom stations and a metaswitch for Hosted PBX.  After trying 2 seperate ACME configuration scenarios (SNAT and PBRB) we have an issue (in both scenarios) where a pilot number in a multiline hunt group is dialed.  Both handsets are behind same nat router.  When one of the extensions picks up to answer, there is a approximately 15-23 seconds of no audio from the handsets before it kicks in.  If the number is dialed directly (instead of a mlhg call) audio passes immediately.  

 

I've attached a summary of the call flow - initially I believed the metaswitch was sending a malformed sdp header - the difference that I saw (between the mlhg call and a direct call) was zeros in the O= line and I cannot verify if the way it is formatted is acceptable.  I am at my wits end - does anyone have any pointers on why it's taking that long for audio (based on my attachment)?