They are legally required to if they fall under FCC jurisdiction.
For entities that fall under the jurisdiction of the Federal Communications Commission (the “FCC”), “[e]ach carrier that offers or bills toll telephone service shall retain for a period of 18 months such records as are necessary to provide . . . billing information . . ..” (47 C.F.R. § 42.6).
https://morningconsult.com/2017/04/25/tech-groups-push-fcc-u...