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