Hello, and thank you for using the ADTRAN Support Community Forums.
Since you have a PRI, and are then breaking out service to the Avaya, I assume you have the ATLAS configured in the DIAL PLAN, with the PRI Service configured as the NETWORK TERM. You can configure "roll over" in the ATLAS, but the ATLAS does all the call routing based on the IN#ACCEPT under the USER TERM. So to have a call roll from one interface to another they will both need to have the same IN#ACCEPT.
So if your main line has "***-7644" as the IN#ACCEPT and you want it to roll to another line when that one is busy, then another line will need to have "***-7644" in its IN#ACCEPT list.
Hope this helps,
Since we haven't heard anything back from you, I'm going to mark this question as "assumed answered". If you have additional questions feel free to continue the discussion.