Abstract: A call waiting service method and apparatus in a voice over Internet Protocol (VoIP) terminal with a public switched telephone network (PSTN) backup function can guarantee at least two VoIP or PSTN calls to a user by providing the second telephone with a waiting tone to inform a user of the second telephone of the fact that a new incoming call is received and setting the third telephone to a waiting call when a VoIP incoming call is received from the third telephone to the second telephone while a VoIP call is performed between the first and second telephones, when a PSTN incoming call is received from the third telephone to the second telephone while a VoIP call is performed between the first and second telephones, and when a VoIP incoming call is received from the third telephone to the second telephone while a PSTN call is performed between the first and second telephones.