What an erlang is
One erlang is one line busy for a whole hour. 300 calls of 3 minutes in the busy hour make 300 × 3 / 60 = 15 Erl. You need more lines than erlangs because calls arrive unevenly: Erlang B finds how many lines keep blocked calls under a target share, usually 1%.
Erlang B or Erlang C
Erlang B is for when a blocked call is lost: a SIP trunk, an E1, channel licences on a PBX. Erlang C is for when a call queues and waits: a call centre or help desk. A call centre needs more people per shift than the formula gives; the shrinkage field accounts for breaks, training and sick days.
For the bandwidth those calls take, use the VoIP bandwidth calculator.