I've never been able to get it to work besides routing the call out TDM. The problem we have is with Contact Center vectoring. They say route to x number whether its a toll free or international number to say india. The vector/VDN is using network region 1 which we tell the ARS to route to our ATT SIP. ATT blocks it because it doesn't have a valid caller ID assigned. ATT is seeing the caller-id of the original caller and isn't part of your ATT DID range. Again, I have never been able to solve this besides routing it out TDM. Basically you need to trombone the call so it goes through the ARS, gets assigned your public unknown. You may be able to solve this by having a virtual extension on CM, have it route the voicemail, and then use Reach Me to route the call to your 800 number. You could also open up a ticket with Avaya to see what they have to say.