Calling number verification is a SIP feature where the calling number is verified by the Internet Service Provider (ISP), and the results of that verification are included with the incoming call. The aim of this feature is to help reduce call spoofing.

• Support for and use of SIP calling number verification is mandated by law for US and Canadian locales. However, you can enable this feature in any locale if the local SIP ISP supports it.
• This feature only verifies the Calling number. The display name information supplied with calls is not verified.

ITSP looks at several factors to verify:

• Is the calling number associated with the subscriber making the call?
• Is the call coming from a known customer?
• Is the call originated by the known ITSP?
• Was the call digitally signed, and could the ITSP able to fetch the public certificate of the originating service provider to verify that the SIP INVITE had not been changed during transit?