Free or discounted call service according to watching of advertisement

Disclosed is an advertising method using a public telephone (100) and a system for providing free or discounted call service according to watching of advertisement. A public telephone (100), a relay server (200), a communication line provider (300), an advertiser server (400b) and an Internet information server (400a) are connected through a communication network (500) together. The relay server (200) receives advertisement information from the advertiser server (400b) or the Internet information server (400a) and displays advertisement information through a display device of the public telephone for advertisement. The public telephone periodically downloads renewed advertisement information from the relay server (200) to renew it. A user is benefited by free-call or discounted-call service depending on seeing and hearing of advertisement over given number. The multifunctional public telephone (100), which is installed at an unspecified location and displays advertisement, provides advertising effect of high level in low price to advertisers.