Deliberately presenting a calling number that the caller is not entitled to use, usually to disguise who is calling or to borrow the trust of a local or familiar number.
In SIP networks the calling number is just a field in the signaling, typically in the From or P-Asserted-Identity header, and the originating equipment can put almost anything there. Spoofing exploits that flexibility. Scammers display local numbers to look like a neighbor, government or bank numbers to look official, or randomly generated numbers to make blocking by number useless.
Not all number substitution is unlawful. Doctors calling from a personal phone, call centers showing a main callback number, and enterprises presenting a central number are common legitimate cases. The Truth in Caller ID Act of 2009 prohibits transmitting misleading or inaccurate caller ID with the intent to defraud, cause harm, or wrongfully obtain anything of value, and later amendments extended it to calls originating outside the United States and to text messages.
STIR/SHAKEN does not make spoofing technically impossible, but it makes it visible. A spoofed call either arrives unsigned, carries a low attestation, or is signed by a provider that can then be identified through traceback. That traceability is what gives the framework its deterrent value, because providers that let spoofed calls onto the network can no longer stay anonymous.
Why it matters to carriers
If your network originates spoofed traffic, it is your signature or your lack of one that will be found during a traceback. Validating that customers present only numbers they are entitled to use, and attesting accordingly, is one of the most direct ways to keep illegal traffic off your network and keep your own reputation intact.