On call during the summer?

I'm new to being an ALT, and I was expecting to have the entire summer off. But apparently I am on call? How does this work?

I'm still paid full for July, but August I'm only paid 65% of my salary so what is the point of working if I'm not being paid full?

Also, can I expect to be told to go to a school or whatever that morning or can I expect something in a week advance? I really want to go traveling.

I heard there are some other events too that are mandatory like English camp but why is it mandatory? My schedule doesn't even have it listed instead its on one of my not on call days.