To provide a telephone number guidance system and its method by which when the telephone number of a speech opposite party is unknown to a server, a new telephone number can be acquired by retrieving an old telephone number registered in a server manager.

The telephone number guidance method includes a step which urges a possessor of a portable telephone whose model is to be revised to register in advance an old telephone number and a new telephone number to a server manager 11, a step where a caller calls an old telephone number of a specific registrant registered to the server manager by using a portable telephone, the caller connects the phone to the Internet 10 when the caller receives a guide message denoting that the old telephone number is disabled of speech from a base station of an old carrier to access the server manager, a step where the caller transmits the old telephone number of the registrant to instruct retrieval to a server computer 12 of the server manager, and a step where a new telephone number of a new carrier of the registrant is transferred to the caller via the Internet based on the result of retrieval.