It is considered "extra work" in our center. All full time drivers*. You are called in senority order. You have to be on the list to be called. They usually call at 6am or a little after to start at 730. *Except there are a couple bid runs in our center that have EAMs built in to their runs. On those runs, if the bid driver is off, a cover or swing driver takes it even though he has less senority.
Some days if you are called in, you can get as much as 13 hours. FWIW, the proctologist for our mgmt team stays busy because they get reocurrent injuries that have to be treated. I know that alot of the feeder drivers at the local hub arent happy unless they get 59.9999999999999 hours a week. It must be true when feeder drivers say they work for a different company. In pkg, that just wears you down, especially in the summer heat. I did almost 2600 hours last year.
I guess alot of it depends on commit times in each area or zip code. They know whats coming in and if something is not on the list when the air shuttle driver is at the airport, he is suppose to call the center so they can call another driver from the list. Sometimes they just get an air driver and thats when grievences are won.