To size the PSTN trunks in a general business configuration, apply the Erlang B model to the PSTN call usage (expressed in Erlangs) and a P01 grade of service. For a call center, a reasonable rule of thumb is to multiply the number of agents by 1.4 to derive the number of trunks. However, a more rigorous approach is to apply the M/M/c/k Finite Queue model.