If I require an employee to call to work in the morning to see if he needs to come in to work that day do I have to compensate him for being on call.

I run a delivery company and as with many companies have employees that call in on their scheduled shift stating they are unable to work on for one reason or another. Due to the nature of our contracts it is a must that we perform every delivery scheduled for that day without exception. For this reason we require 2 individuals (on a staff rotation to keep it un-bias) to be on-call each morning. They are asked to call the office at 7:30am to see if we need them to work that day. Do I have to compensate the employees for calling in to see if we need them to work?