Embodiments of the present invention relate to communications technologies and disclose a method and a system for establishing a data transmission channel, a SIP server, and a session border controller. A first authentication request message is sent by a first terminal. The first authentication request message includes identifier information of the first terminal and identity information of a second terminal. It is determined that the identifier information of the first terminal matches pre-stored authentication information of the first terminal. According to the identity information of the second terminal, a second authentication request message that carries authentication information of the second terminal is sent to the second terminal. After receiving an authentication success response message from the second terminal, a data transmission channel is established between the first terminal and the second terminal.