This is a set expression. To "go above and beyond the call of duty" means to do a lot more than you are required or expected to to for your job. If you break the expression down into parts, "go above and beyond" means to do more. "The call of duty" means "the things that you're asked to do for your job". This expression comes from the military, where it was used to talk about soldiers who died while fighting for their country.
Now, the phrase is often used to talk about teachers, social workers, nurses, and other people who work hard to help people. It can also be used to talk about customer service:
We go above and beyond the call of duty to ensure that every customer is 100% satisfied.
As you can see from the example at top, you can also leave off the second part and just say:
They really go above and beyond.
Whether you include "the call of duty" or not, this is a pretty formal phrase. It fits best in written English or in formal speeches.