1. present: I am just calling. (Speaker's viewpoint is that what he is doing at the moment of speaking is "calling".) 2. past or present perfect: I just called. I have just called. (Speaker's viewpoint is that what he is doing at the moment of speaking is not "calling", but "speaking on the phone". The "calling" occurred a moment earlier.)
The example above shows "Shakespeare" and "greatest poet of all times" for the A and B. You could also say something like "I consider Meg to be my best friend." "Meg" and "my best friend" are the A and B.
In my example "the act of dialing" and "calling" are A and B - because were were talking about how people define the word "calling."