* "Caller Verified is T-Mobile’s implementation of the STIR and SHAKEN standards" [0]
* "Verizon reiterates both our commitment to deploying the STIR/SHAKEN call authentication technology" [1]
* "Sprint supports efforts to build the SHAKEN/STIR call authentication system" [2]
* AT&T doesn't seem to mention STIR or SHAKEN by name on their robocall page [3] but the FCC sent them a letter thanking them for their "commitment to implement a robust call authentication framework in 2019" [4]
* "Comcast is proud to be at the forefront of efforts to address the scourge of illegal spoofed robocalls in this country, including the development of the end-to-end call authentication protocol known as SHAKEN and STIR" [5]
etc
[0]: https://www.t-mobile.com/news/caller-verified-note9
[1]: https://ecfsapi.fcc.gov/file/1119062987519/2018%2011%2019%20...
[2]: https://ecfsapi.fcc.gov/file/1091391175368/Sprint%20Call%20A...
[3]: https://about.att.com/sites/cyberaware/ae/robocall
[4]: https://docs.fcc.gov/public/attachments/DOC-354933A2.pdf
[5]: https://ecfsapi.fcc.gov/file/111974588343/Comcast%20Resp%20t...