Some outbound calls die before making it outside

Every once in a while (2-3 times a day), someone will attempt to make an outbound call through our PRI and before the ringing begins, the Congestion sound is played.

I get this on the console:

-- Executing Dial("SIP/awilliams-cda3", "Zap/G3/17153999900") in new stack -- Requested transfer capability: 0x00 - SPEECH -- Called G3/17153999900 -- Zap/23-1 is proceeding passing it to SIP/awilliams-cda3 -- Zap/23-1 is making progress passing it to SIP/awilliams-cda3 -- PROGRESS with cause code 17 received -- PROGRESS with 'user busy' received, signaling AST_CONTROL_BUSY instead of AST_CONTROL_PROGRESS -- Zap/23-1 is busy -- Hungup 'Zap/23-1' == Everyone is busy/congested at this time (1:1/0/0) -- Executing Congestion("SIP/awilliams-cda3", "") in new stack

The phone company said that they saw that the PRI channel was busy, They turned on something that gave us the “PROGRESS” error messages. Before this, the console would just claim that we received a hungup from the other end

Why is this happening? We are dialing from the bottom of the PRI while incoming calls enter the top of the PRI, so it isn’t a collision between inbound/outbound (we don’t have more that 3 inbound and 4 outbound at any time). Any ideas?

Post a pri debug