42 C.F.R. § 438.52
(a) General rule. Except as specified in paragraphs (b) and (c) of this section, a State that requires Medicaid beneficiaries to:
(b) Exception for rural area residents.
(1) Under any managed care program authorized by any of the following, and subject to the requirements of paragraph (b)(2) of this section, a State may limit a rural area resident to a single MCO, PIHP, or PAHP:
(2) To comply with this paragraph (b), a State, must permit the beneficiary—
(ii) To obtain services from any other provider under any of the following circumstances:
(B) The provider is not part of the network, but is the main source of a service to the beneficiary, provided that—
(1) The provider is given the opportunity to become a participating provider under the same requirements for participation in the MCO, PIHP, or PAHP network as other network providers of that type.
(2) If the provider chooses not to join the network, or does not meet the necessary qualification requirements to join, the enrollee will be transitioned to a participating provider within 60 calendar days (after being given an opportunity to select a provider who participates).
(c) Exception for certain health insuring organizations (HIOs). The State may limit beneficiaries to a single HIO if—