If you have just one E1 or T1, then the effective traffic is given by BHCC calculation (# of completed calls * Mean holding time / 3600). Note that BHCA=BHCC.
Traffic Offered is calculating the number of erlangs using Erlang B table based on the number of circuits available in the TG and GOS. Note 1 TG not ever is 1 E1 or 1 T1.