Most companies, depending on the size, have 1 person at a time on call, therefore I would create a hunt group for all on call personnel, make sure HG extension is a trusted number under the source tab of all users in the group. Add pager/cell number as a trusted number for each user as well, and put ringback on VM for that hunt group. Build a callback start point for that group with "get mail" action in VM Pro with HG name/extension as the destination. Condition editors can be used for times of day/days of the week to notify the individual on call for that day, which would require conditioner editors, test conditions, and "get mail" action for each member and hunt group name/number in the destination tab. It will probably require more than one condition editor/test condition to accomplish this.
You can also try the simple route of having the users disable themselves when they aren't on call. My theory is it should only page the active/available members in the group.
Hope this helps.