If you have service with an MVNO that disappears one day, can you still port your number to another provider?
Do MVNOs underlying carrier (AT&T, Verizon, etc.) maintain account number and PIN port-out information for end-user customers of MVNOs directly in their own database so that they can process port-outs even if the MVNO disappears?
Do FCC regulations require an MVNO to give a certain number of days notice prior to discontinuing service?
If you want an authoritative answer to these questions, the FCC would provide that. Almost certainly, that information would be given in response to a simple, clearly-formulated, question and would not involve resorting to the provisions of the FOIA.
The FCC would provide the regulatory response to the last question, but likely not the first questions. And even the answer to the last question might be the correct regulatory response, it may not necessarily be how companies have historically acted perhaps in violation of their regulatory obligation.
And, if your number has already been assigned to someone else after MVNO's overnight disappearance, do you or the new assignee own it at that time? Whether "it" can be owned is a matter of state law, thus varies accordingly.