Set up one phone number for IT support?

I am trying to figure out a solution that I can give out one phone number to all company employees that will then call my staff’s cell numbers in a particular order depending on who is on call or not? I have found one company that does it www.grandcentral.com. They even have a free service with 100 minutes a month. Does anybody else know of other services out there free or fee based that is comparable?

Thank you for your time.

Ryan