It is funny that something so widely used can become difficult to find. We use erlangs all the time to estimate the trunks, loads, agents etc; but usually through a software like phonecalc etc.
Anyways the best info on this is available at Wikipedia (BTW this is fast becoming my seond favorite site for info-search after Google)
Follow this link:
This site has an online calculator to give a good output estimate. But if you need the equation then it is available at wikipedia.
I hope this helps.