Hi Guest!
Great question. Some medical insurers will require that you complete credentialing activities/paperwork in order to bill as an out-of-network provider. Some will simply require you to submit a copy of your W-9 with you first claim or few.
For example, many BCBS insurers will require that you register as an out-of-network provider - many have the registration form available on their website. Some will require that you have a CAQH profile, so if you don't have one just yet, that's a great place to start! Here is a link to the CAQH register/login page:
https://proview.caqh.org/Login/IndexHope this helps!