The present invention is directed to a method for placing a call between a client in one network and a client in another network. The IP network includes a SIP server and a network gateway configured to provide access to a public switched network. The method includes receiving a SIP call request message from the first client. The SIP call request message is authenticated to thereby identify an authentic originating client. Subsequently, a database is searched to find client billing tag corresponding to the authentic originating client. The call is completed if the client billing tag is obtained, and not completed if the client billing tag cannot be obtained. Thus, the present invention provides an efficient method for billing phone calls that are placed from SIP enabled devices to a telephone connected to the Public Switched Telephone Network (PSTN). The method of the present invention also substantially eliminates certain types of fraud.