[Attribute Name] callerid [Details] 
is an optional ECMAScript expression which returns a string defining the caller identity to be used when making the outbound connection. The format of this information is protocol and platform specific but might consist of a telephone URI, as described in http://www.ietf.org/rfc/rfc2806.txt or a SIP URI as described in http://www.ietf.org/rfc/rfc2543.txt. Note: An implementation platform is not required to use the specified data and certain protocols and network elements may prevent it's use. If the platform does not support specification of callerid it should raise a connection.PROGRESSING event and indicate that the use of callerid is not supported.