Implements different formulae used to compute the delay probability and the service level, assuming some queueing model for the arrival and service rates. Examples of models are the Erlang $A$, the Erlang $B$ and the Erlang $C$ models. The service level is defined as the probability \textit{Pr}\{$W \le$ \texttt{awt}\}, where $W$ is the waiting time and \texttt{awt} is the acceptable waiting time.