The Telephony Application Programming Interface refers to a programming interface for telephony applications. Via TAPI it is possible to integrate local telephone numbers of a location into the call distribution. Calls can thus be transferred to the extensions of free employees at the same location without duplicate channel assignments and additional transfer costs. The calls are transferred by the system via TAPI without call forwarding. The TAPI function also enables 'busy detection' for transfers and makes outbound calls possible via an ACD system with the local PBX at flat rate conditions.