A preliminary verification method under an Internet environment using a phone authentication service in a system including a first terminal, a web server, an authentication server, a host server, a call server and a second terminal, the method comprising the steps of: providing a preliminary transaction request generated from the first terminal to the authentication server through the web server; at the authentication server, transmitting the preliminary transaction request information to the host server, and providing preliminary transaction result information corresponding to the preliminary transaction request information from the host server to the first terminal; at the authentication server, in case of receiving a real transaction request generated from the first terminal, receiving and verifying the authentication information from the first terminal according to the electronic transaction approval, and requesting a phone authentication for the second terminal of the user of the authenticated first terminal to the call server; at the call server, generating an authentication call to the second terminal, receiving a one time password (OTP) number corresponding to the authentication call from the second terminal, generating an OTP number by the same number of an OTP device, and transmitting authentication completion information to the host server if the generated OTP number is the same with the received OTP number; and at the host server, performing the electronic transaction process according to the authentication completion information received from the call server.