Hi David
Interrupt Service Routines (ISR) are never called by us, we just define them; I mean these are interrupts and they can occur anytime during the code. They are generally triggered automatically either by internal functioning (software interrupts) or maybe due to some outside hardware changes (external interrupts).
Most of the interrupts need a corresponding flag bit to be set (logic1) in order to jump over to their ISR and all this happens automatically, we can either just monitor these flag bits (which is termed as polling technique) or set up an ISR in order to execute a funtion on its own whenever interrupt occurs.
Remember, the global interrupt enable bit should be set too to make all this happen.