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Outbound SIP random fast busy

Lexmar asked
Last Modified: 2013-12-21
This is one of those who's on first kind of issues.

I have a commercial asterisk box in an outbound call center.  About three weeks ago the agents making calls started getting about 25% of their outbound calls playing a recall (Fast Busy) when they are running hard.  This system is 100% SIP with no cards and running G711.  There is a dedicated T-1 for VOIP only and the trunk is set for 18 concurrent calls.  Other internet traffic is handled entirely by an 18x2 Cable Internet connection on a dual WAN router.  Problem happens when there are about 13 calls going but can be as few as 6.

On wireshark the asterisk box is sending a 486 Busy Here signal to the extension.  On the same call the SIP provider logs show a status code 404 Not Found.  I can't confirm the 404 yet as the switch the asterisk box is on doesn't support sniffing.  I have them relocating the PBX in the call center to allow me to run Wireshark on it's full packet stream.

I had the SIP provider run diagnostics on 565 incomplete calls last Saturday (3500+ completed) and all the 404 calls showed only after the 3rd gateway was given the invite to handle the call as the primary and secondary gateways were busy and declined the call.

My thought is that I am up against a timer setting somewhere that says if the call hasn't connected in X milliseconds hang up?  Does anyone know if asterisk has a setting for time to connect and if so where is it - which conf file?
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