I don't think so, with the "without touching the NEC" restriction. Disabling call-waiting in the NEC should do it.
This could also be a cause code mapping issue. Debug ISDN l2-formatted and place a call to 111-SHOP. With this call up place a second one. You should see a cause code 17 user busy from the PBX. The TA900 should map this to 486 busy here and the provider should return a busy signal.
If the PBX is returning something other than 17 user busy for a busy extension, then you need to map that cause code to 486 busy here in the TA900.
There might be another way. I haven't tried it. It's kind of an ugly hack, but it might work.
- Create a second ISDN trunk referencing the same PRI to the NEC
- Set the maximum B-channels to "1"
- Create a grouped-trunk containing only 111-SHOP and include that trunk. Exclude 111-SHOP from the main ISDN trunk group.
There are support issues on the customer NEC switch so we're being prudent in recommending changes unless absolutely needed. Clearly disabling call waiting should resolve the issue - we'll start there with the customer, and use the other two approaches as backup. We have access to other PBXs to test these changes with the Adtran ...
Many thanks for the response - this feedback helps us with next steps.
All the best