Here are a couple of links that will give you some background to Erlang. I thought I had a copy of the formula but I can’t seem to put my hands on it right now. If I do I will post it.
http://www.teleopti.com/callcenter/wizard/about.asp
http://www.harcourt.com/dictionary/def/3/6/4/1/3641300.html
http://www.abc.net.au/science/k2/moments/s110494.htm