What's the target line for your main number?
If it's target line 241 for example, make the DN that will activate the service the control set for that line. Delete all sets from ring group 1 (otherwise any other DID's will go to the night service). Remove the default DN from ring group 2, add the voicemail DN as the only set in ring group 2. Configure line 241 as a member of ring group 2, start the service, and calls to line 241 should go directly to auto attendant.