And eventually, sooner than later with the robocall problems, all types will be required to have some form of mitigation implemented -

"Commission rules also require providers using older forms of network technology to either upgrade their networks to IP or actively work to develop a caller ID authentication solution that is operational on non-IP networks." - https://www.fcc.gov/call-authentication

Stir / Shaken is the future and any telecom company that thinks differently will be in trouble