John has issues with coverage in his home town. Leo says that the irony is that AT&T is considered to have total universal coverage, but that doesn't seem to be the case entirely. Leo says to go to the carrier he's with and ask them for a FemToCel. If they want to keep his business, they'll give him one for free. With it, it will route his calls over the internet for cell service to his phone.
He could also get a phone that has dual SIMs, and OnePlus makes one. Or he can go with Google Fi, which switches carriers depending on what carrier is the best where he's at. It's pay as you go, too. The only drag is that he can only use select Google brand phones and that means buying a new one at full price.