in isr there is no return value but in function call there 
is return value

In function call the main program and the funtion have some 
relations but in ISR there is no need for this so there is 
no need of return value in ISR.

In case of a function call the arguments,local variables and
return address is stored in the stack where as in case of an
ISR, after executing the current instruction the context is
saved with no return value.

isr will be executed only when the system is interrupted 
whereas the function call meant for any operations to 
perform eventhough when there is no interrupt