was telling, told, have told, have been telling

[joham]

This is Mr Carter, who I was telling you about.

We often introduce someone to the other person this way. I'd just like to know why we can't say 'This is Mr Carter, who I told/ have told/ have been telling you about. And if we can say the sentence in these other ways, what are the differences between them?

Thank you very much in advance.

[rewboss]

In fact, for most of your sentences, the meaning is almost the same, and you could use any of them. But there some very small differences:

...who I was telling you about. (At some time in the past, I was telling you about him, until we were interrupted/changed the subject/went home...)

...who I told you about. (I told you about him in the past. This is very neutral.)

...who I have told you about. (You already know something about him.)

The last one is a little different:

...who I have been telling you about. (He is the subject of the conversation which we are having now. Or: I have told you many things about him repeatedly over a long period of time.)

[joham]

Rewboss,

I don't know how to thank you enough. You've cleared my puzzles for a long period of time. Thank you for your kindness.

joham