About the different engineering methods:

Three engineering methods are commonly used in telephone traffic engineering. By-in-large, the differences between them are very subtle as you can see by selecting the different methods while keeping traffic constant.

Erlang B -

Also called the Lost Calls Cleared method. A mathematical model of telephone traffic in which blocked calls are not queued. For example, incoming DID calls which exceed the number of DID Trunks, are returned busies by the central office and the caller hangs up.

Erlang C -

Also called the Lost Calls Delayed method. In this case, blocked calls do not disappear, but are queued. Many outgoing COT trunk groups, and particularly ACD Queues, employee queuing.

Poisson -

AKA the Lost Calls Held model. In this case, blocked calls are held for their normal holding times, even though unserviced. This method is commonly used for internal system resources, such as requests for internal dial tone, busy tones, etc. It may also be used for manually terminated Tie Lines, requiring operator assistance.