Medicare Administrative Contractors (MACs) for your state will be the primary source of contact in the process.
Here is a list of MACs by state and region:
Medicare Administrative Contractors (MACs) are private health care insurers that has been awarded a geographic jurisdiction by the Centers for Medicare and Medicaid Services (CMS) to process Medicare Part A and Part B medical care and mental health claims for Medicare Fee-For-Service (FFS) beneficiaries. CMS relies on a network of MACs to serve as the primary operational (intermediary) contact between the Medicare FFS program and the providers enrolled in the program. MACs are multi-state, regional contractors responsible for administering both Medicare Part A and Medicare Part B claims.
Key services that MACs perform include:
Providers enroll in Medicare through their designated MAC. You can find your designated MAC and their contact information and mailing address here: https://www.cms.gov/medicare/provider-enrollment-and-certification/medicareprovidersupenroll/downloads/contact_list.pdf