The present invention relates to a PBX system for providing exchange service to a number of extension subscribers. In the PBX system, a main exchange center provides a private branch exchange service to a number of extension subscribers. When a ring is received via a main line, the main exchange center judges whether the ring is received from an SMSC, and if the ring is received from the SMSC, notifies the receipt of an SMS message. An SMS processor receives the SMS message from the SMSC via the main exchange center, extracts an originating or destination unit data therefrom, refers to previously stored extension unit-matching information to search for at least one destination extension unit which the SMS message is headed for, and distributes the SMS message to the destination extension unit.