Hello @KRosenthal, thank you for reaching out to us here at The Community, my name is Sherissa and I am more than happy to assist you today.
First, you will want to make sure you have your Payroll Item Mapping set up properly, here is the link with a video on how to accomplish that; https://intuit.me/2TkxXxD
Once that is set up the users will clock in/out & switch between Customers/Jobs & Service Items as normal. Whenever the user has tracked time against the "On-Call" option they will be given the pay rate that you set up against "On-Call" in the Payroll Item Mapping Tool.
I hope this helps if you need anything else please don't hesitate to reach back out to us here at The Community.