12:41 Nov 11, 2011 

I think that is a very fine distinction that is not appreciated/applied by many people. In practice the terms are virtually synonymous. And in its most general sense of "to bring into action", invoke can be used in either context.

12:00 Nov 11, 2011 

in programming, you call a function or routine, but invoke a service. The mechanism IS different, functions or routines are part of the program and run on the same machine, services are separate programs that may be in a different machine.
Calling more or less implies waiting for an answer which you won't want to do when it may run on another continent.

11:47 Nov 11, 2011 

Just out of interest, although both proposed terms (call and invoke) are correct, I would actually not user either in other contexts such as on a user interface (i.e. not for technicians).