The present invention provides a “Phone Log” in a telephone switchboard for recording the extension numbers, the phone numbers dialed by the extension numbers and the accumulated times. When there is an outside call, the telephone switchboard will first check the “Phone Log” whether the telephone number of the outside call is on the “Phone Log” or not; if the telephone number of the outside call is on the “Phone Log”, then the outside call (callback) is connected directly to the corresponding extension; otherwise the outside call will be connected to an auto-attendant for connecting. As the accumulated time of each of the records reaches a time limit, the related record will be cleared out. After the callback is picked up by the corresponding extension successfully, the related record is also cleared out.