A conferencing system that can access advanced conferencing features while following essentially the same call flow as conventional conferencing systems. The conferencing system includes a computer network, and at least one conferencing application server, at least one media server, and at least one user agent connected to the network. The conferencing application server establishes and manages multimedia conferences by engaging in Session Initiation Protocol (SIP) signaling with the user agents and the media server. Once the conference is established, the media server generates multimedia data such as audio data and conveys the data to the conference participants. In order to access advanced conferencing features, the conferencing system employs an enhanced SIP signaling technique including a conferencing Application Programming Interface (API) implemented by incorporating Extensible Mark-up Language (XML) messages in the bodies of respective SIP request/response messages. The XML messages are incorporated in the SIP request/response message bodies to convey conference specific commands and/or parameters that cannot be easily described via the Session Description Protocol (SDP).