A method for establishing a multimedia connection with quality of service (QoS) using an ATM backbone for ISDN network or IP network calls. QoS makes IP over ATM useful for video and audio conferencing by ensuring a minimum packet error rate at acceptable bit transmission rates. The invention will utilize the H.323 annex C protocol to transmit IP gateway transmissions through an ATM backbone network. This invention enables H.320 endpoints having a connection to an ATM network to send QoS communications to other endpoints. When call setup is initiated between two endpoints, an ISDN (or IP) call is sent to a gateway. The gateway will utilize a network gatekeeper to route the call to the addressed endpoint's corresponding gateway. When a route is established, the call will be converted to an H.323 annex C protocol transmission and sent through the ATM backbone to the destination gateway. At the destination gateway, the call will be converted to another protocol (as necessary) for delivery to the destination endpoint.