Service Company Rotating Extension On-Call

Not sure where to start. We are about to implement Asterisk. We are a technical service company with 24hr On-Call support. We currently use an answering service but, want to replace that with an automated attendant.
Currently at 5pm the office call forwards line 1 (of 4) to the answering service and line 3 to the cell of the guy on call. At 8am the following morning they remove the forward. Man on call has a backup and sometimes the on call pair varies.
How can we set this up so that when someone calls in and presses 1 for an emergency support issue the calls work as described above and yet still make it simple enough so that a non-technical office person can accomplish it?

We use vm notify pro http://www.schmoozecom.com/vm-notify.php for this purpose