Marriott is looking ahead. The STIR/SHAKEN system for tracing robocalls is going live by June 30, 2021.[1] It's like TLS certs for phone companies. Voice call source spoofing is about to get much harder. A few robocall companies have already been caught. Marriot is filing a case which will allow them to subpoena the data which tells them who to sue.
[1] https://www.fcc.gov/TRACEDAct