Are state agencies required to pay "hourly" employees for being on-call? Being on-call, we are supplied department cell phones and are required to remain in a geographic location and to respond to all calls immediately.
Thank you in advance for your assistance and expertise.

Whether any employee is required to be compensated for on-call time is going to be very situation specific. It's going to depend on how large is the geographic area you are confined to and what is meant by immediately. Within five minutes? Fifteen minutes? Half an hour?

Consult your state DOL.