Section 202 of Telecommunications Act details that telecoms cannot discriminate connections so they have been unwilling to block spam if it can lead to breaking the law[1]. The FCC has been trying to change that by rolling out new guidance on what can definitely be blocked[2]. One of the new rules defines a new legal safe harbor for carriers that block calls that are unauthenticated using a new protocol that is expected to be rolled out by year end, 2019.
[1]: https://www.law.cornell.edu/uscode/text/47/202 [2]: https://www.fcc.gov/document/fcc-affirms-robocall-blocking-d...