Here's what I would do-
In Administrator, create a Schedule for each of the On-Call people, and set it to their next scheduled on-call.
Switch to Attendant, and create a System schedule for each of your On-Call people in your On-Call profile, set to their Administrator schedule. Set the Attendant schedule to route the call to their number.
Now, each time one of your people completes an On-Call rotation, set their Schedule in Administrator to their next scheduled On-Call.