RVgo4it
If you go here and put in his area code and his exchange, you'll be able to see what carriers serve the area.
https://www.telcodata.us/search-area-code-exchange-detail
For EXAMPLE if you enter 207-528 which is for the rural town of Patten, Maine, you'll see these companies:
CONSOLIDATED COMMUNICATIONS
(the local phone company, hence designated ICO)
(in some other situations the local company might be designated as ILEC or in legacy Bell territory as RBOC)
TIME WARNER CABLE
(this is designated as a CLEC, yes, but presumably for Charter/Spectrum cable phone service, as Time Warner Cable is now Charter).
NEW CINGULAR WIRELESS
(this of course is AT&T Wireless)
So in our EXAMPLE, you can see that there does not seem to be any carrier in the local exchange area for a VoIP provider to utilitze.
Sometimes arrangements can be with a carrier in a nearby exchange in the same rate center---I'm not at all an expert in that.
EDIT: I have NO IDEA why the forum software is highlighting some of my words in RED....