Approximation of a M/M/c queueing model with the Erlang A, B and C formulae. The model assumes a Poisson arrival, exponential service time and patience time. In a loss model, the model assumes no waiting queue. In the delay model, a call can abandon if its waiting time is greater than its patience time. For the Erlang C formula, we can assume an infinite queue capacity (due to tractability of the formula) or a limited capacity. For the Erlang A formula, the user must give a limited capacity to the queue.