What natural monopoly? I have a choice of mobile phone service providers, so you can’t have meant that. If any of the phone companies had made themselves free, I’d ask if they really had a viable business or if they were instead trying to undermine any potential competitors in order to create a monopoly.

The US has 4 mobile network providers. Sprint is useless, Tmobile is okay in certain areas, but really, the 2 with nationwide coverage are ATT and Verizon. In my experience, Verizon coverage is more solid than ATT, but in general these are the 2 networks that provide the most comprehensive coverage.

A natural monopoly exists anywhere there are extremely high infrastructure costs, such as running pipes and wires to every home. The same thing exists with mobile networks, as it requires towers everywhere.

The towers can usefully overlap, in a way that pipes and cables don’t, so I disagree with you about it counting as a monopoly.

And that’s without roaming, which I don’t understand, as it means I have cheaper mobile minutes/texts/data in the USA and Germany than most locals.