Can Erlang Help Me?

I am new to this field, and do not know Erlangs equation. Can any one recomend some reading on the formula(e)?

What I am really trying to solve:

I know my port usage at time t and hold time. I am trying to develop a formula to determine the new call arival rate at any time t as a function of these parameters. I am trying to solve this for infinite port count and will ceiling the function based on total availible ports.

Thanks

Bob