To create a new label, go to MANAGE TIMEBANDS and create/edit a timeband.
In the NAME enter the label you want to use, like ON CALL.
Set the start and end time to the same time (ex. 9am)
Go to the schedule screen and you will see the new timeband.
This timeband will always show the name vs the time since it is 0hrs long:
You can now drag and drop this shift into the schedule: