Your Auto Attendant, is this a Cisco Unity server by any chance or the built in Auto Attendant (AA)? If you are using the built in AA, the trick is to set up a Hunt List, so there are alternate extensions to go to, if the first number is busy. As far how to do this I would recomend reading the administration guide. Select which version you have here;
To get the answers on all your CallManager questions, (from the horses mouth) go to http://forum.cisco.com/eforum/servlet/NetProf?page=main This is the Cisco forum site. Then go under IP Telephony.
Let me know if you have any other questions.