Basically, CallXML is used to describe the user interface of a telephone, VoIP, or multimedia application to a CallXML browser. A CallXML browser then uses that description to control and react to the call itself. According to a Voxeo, a vendor that supports CallXML, the markup language includes:

• Media action elements such as "playAudio" and "recordAudio" to describe what to be presented to the user during a call
• Call action elements such as "answer", "call" and "hangup" to describe how to control and route the call
• Logic action elements such as "assign," "clear" and "goto" to describe how to modify variables and interact with traditional server-side Web logic such as Perl, other CGI languages, PHP or ASP
• Event elements such as "onTermDigit," "onHangup" to describe how to react to things the user can do during the call, such as pressing digits or hanging up
• Block elements that logically group actions & events together, so that one set of event handling elements can be used for several sequential actions