47 C.F.R. § 64.6050
Billing-related call blocking
No Provider shall prohibit or prevent completion of a Collect Calling IPCS communication or decline to establish or otherwise degrade any Collect Calling IPCS communication solely for the reason that it lacks a billing relationship with the called party's communications service provider unless the Provider offers Debit Calling, Prepaid Calling, or Prepaid Collect Calling for IPCS communications.