An emergency calling system and method via a cable modem includes a cable modem connected to a Public Switched Telephone Network (PSTN) backup line in a cable modem-based Internet telephone service to transmit the emergency telephone number inputted by a subscriber telephone to the PSTN via the PSTN backup line. This enables an emergency calling function via a PSTN backup circuit and stable PSTN in a cable modem-based Internet telephone service. This also allows a cable modem-based Internet telephone service to promptly provide urgent emergency calling via the PSTN line in case of an emergency.