https://techcrunch.com/2019/08/22/us-cell-carriers-team-up-to-combat-robocalls-but-no-deadline-set/ > Twelve cell carriers, including the four largest — AT&T, Sprint, T-Mobile, and Verizon — have promised to make efforts to prevent spoofed and automated robocalls. > Announced Thursday, the pledge comes after 51 U.S. attorneys general brokered a deal that would see the telecom giants roll out anti-robocalling technologies, including a way of cryptographically signing callers to wipe out phone number spoofing. Known as STIR/SHAKEN, the system relies on every customer phone number having a unique digital signature which, when checked against the cell networks, validates that a caller is real. The carrier near-instantly invisibly approves the call and patches it through to the recipient.