[Name] conferenceid [Required] false [Attribute Constraints] Must not be used with connectionid [Type] ECMAScript Expression [Default Value] none [Valid Values] Conference IDs [Description] 
An Optional ECMAScript expression which returns the identifier of a conference bridge. If specified, the dialog being prepared MUST be joined to the referenced conference as specified by the mediadirection attribute. If the attribute value is invalid an error.semantic event must be thrown.