CS2100 + AS5300 SIP Trunk

[opethPA]

Hey All..

We have an AS5300 with a SIP trunk going to a Cisco CM 8.6.
We are able call over that SIP trunk from the Avaya to the Cisco without issue.
When we call the other way we either get a fast busy or a standard SIP code of 503.
This makes me think that I have something configured incorrectly on the 5300.
Does anyone have a PDF , guide or has setup a SIP trunk in a similar evt?

 

[opethPA]

some additional info...
When I look at the log on the Session Server Manager I see the following

Jul 6 10:06:24 SST-01 sipgwyappln: SIPC731 NONE INFO SST Calls Rejected 
Location: SST-01 IP Address: 10.X.X.X (ip addresses of CS2100/As5300)
Time: Fri Jul 6 10:06:24 2012 
Component Id: SST-01 
Unit Number: 1 
Description: Incoming Call from SIP Server CCMSUBPW5 Rejected With 503. REASON: Failed to select SIPLINK

The scenario again is when I place a call from the Avaya to a Cisco Call Manager over the SIP trunk everything works great.

When I place a call from the Cisco Call Manager to the Avaya I get a fast busy.