The most fundamental function of a PBX system is to support switched connections between peripheral endpoints. Stations users are accustomed to picking up their handset, hearing the dial tone, dialing a telephone number, and being connected to the called party. The possibility always exists that the station user receives a busy signal when the dialing process is completed. The most probable reason for a busy signal is that the called party is off-hook and engaged in another call. Infrequently, all telephone company trunk circuits are busy, and the station user hears an announcement to call again at a later time. A busy signal also may be received when all PBX trunk circuits are in use or internal switch network resources are not available. The PBX station user cannot control the availability of the called party or the availability of PSTN trunking facilities but can minimize the probability of busy signals due to blocked access to the internal switch network or local trunk circuits, if the PBX system is properly configured and engineered to meet the expected traffic demands of the customer. PBX traffic analysis and engineering tools are used to achieve acceptable customer service standards for internal switched connections and off-premises trunk calls.