In the past, well worn methods of traffic engineering were used to plan voice networks. Calculations such as Erlang B and Erlang C ruled the day. Since T1's were used, companies typically added circuits in blocks of 24. SIP changes all this. The key metric of SIP traffic engineering is the number of concurrent calls needed. This not only equates to the way the service providers bill for PSTN SIP circuits, it relates directly to bandwidth required.To properly engineer the network at each phase of the deployment, you need to calculate the number of concurrent voice channels needed. These calculations must be made for both on-net and PSTN calling. Many companies simplify this by guessing. For example, if a location had four POTS lines, they simply assume they will need four SIP circuits from their service provider. This can be a costly mistake.