SIP Status 491 Request Pending Then SIP status 481 Call Leg

I am troubleshooting random call drops with a new Polycom RealPresence Group 500 unit with our asterisk 1.8 build. I have about 100+ polycom ip 335 deskphones connected in the office however this new AV equipment with SIP always having issues dropping calls. I ran some packet captures from the serverside and the unit, what I notice from the server side is before the call drops. I can see asterisk sending ACK back to the polycom when it sending 491 request pending packets. Eventually I see Asterisk sending INVITE SDP packet and then the SIP msg 481 Call Leg/Transaction Does not Exist. INVITE and SIP 481 happens for a little while then eventually the Polycom unit sends a BYE to the asterisk server. However on the Polycom physical unit it still thinks it’s on a call. Later on all I see in the packet capture are PAYLOAD [Block ID : xxxx] messages.

Anyone seen this before or can point me in the right direction in troubleshooting this?

I’d suggest providing a complete SIP trace using “sip set debug on” so we can see the full flow of signaling.