SIP Number Portability Parameters

http://www.voip-info.org/wiki/view/SIP+Number+Portability+Parameters

http://www.dialogic.com/webhelp/img1010/10.3.3er2/webhelp/Description/Interworking/SIP_Carrier_Identification_Code.htm

http://www.voip-info.org/wiki/view/SIP+URI