I have a 7100 that has been configured for several years using analog lines and a scheduled system mode change to send calls to specific Auto Attendants based on the time of day. I am attempting to establish a new SIP trunk with a new carrier. From my research I believe the best way to accomplish this is to configure the DID of the SIP trunk on a new Virtual user account, set the user to not right and let call coverage handle the distribution of the calls (mirroring the configuration of the old analog trunks). When running debugs of SIp Stack Messages and voice verbose; I can see T08 (my new SIP Trunk) attempting to hand the call to 821 (AA for day calls). I just get a busy tone from the call.
Sanatized my debug outputs; 4843000000 is the cell calling in; 6108888888 is the DID on the system; SIPCARRIER is the IP of the Carriers SIP Server; SIPCARRIERRETURN is another IP of the Carrier.
I would configure a single ring-group that only has coverage to the auto-attendants. Then assign every DID to that group where calls could come in to the system. Using this method all calls regardless of the carrier will be handled the same way. Also you could have an issue where multiple calls come in at the same time to the virtual user resulting in more calls then that user can handle causing a busy issue as well. Also virtual users count against your user license limit where ring groups do not, especially important if using the SMB version of the 7100. I am guessing it maybe a class of service setting causing the problem where the virtual user may not have the ability to transfer to the AA, or a codec issue, but without the rest of the config it is a guess.
Something similar to this:
voice ring-group 807
coverage aa 821
coverage night aa 822