There's a big difference between on-call teachers and supply teachers.
Supply teachers are part-time teachers whose job it is to fill in for absent teachers.
They go from class to class and they are generally qualified to teach the subject they are supplying. Unfortunately, this means that you could end up with a phys ed teacher in your math class. Now most students groan when they walk into a class and see a supply teacher. The stories about them are endless and there are plenty of jokes. But now, many students would be glad to see supply teachers back in the classroom.