traffic in PSTN

Can you tell me (or tell me where can I find it) and describe how can I calculate telephone traffic in Erlang (E) if I know number of subscribers,number of incoming and outgoing trunks and I know advanced traffic by subscriber (0.08 E).
Outgoing traffic is equal to incoming.
Where can I find Erlang’s table and how can I use it?
Thank you and I hope to get your answer soon,

Zaklina