Internet Telephony Service Providers (ITSPs) implement the calling number verification using several SIP RFCs, collectively referred to as STIR/SHAKEN.
STIR
The Secure Telephony Identity Revisited (STIR) protocol uses digital certificates between the customer (the call originator) and the ITSP to establish customer authentication. The ISP can examine known numbers allocated to that customer for number authentication.
SHAKEN
Signature-based Handling of Asserted Information using toKENs (SHAKEN) are guidelines for PSTN network providers handling calls that transit from the non-SIP PSTN to SIP networks. Currently, it has mainly been implemented as a service for SS7 carriers in the USA and Canada.
