An Erlang is the amount of traffic it takes to keep a traffic sensitive facility busy for one hour.
If I have 1 Erlang of traffic with a smooth arrival pattern, I would only use 1 trunk. The 1 Erlang of traffic can be 1 call for 1 hour or 20 calls at 3 minutes each one after the other.
I would suggest reading the following:
http://www.cisco.com/univercd/cc/td/doc/cisintwk/intsolns/voipsol/ta_isd.htm#42424