This likely differs a lot between countries (though there should be a common core due to ITU of what is being regulated how by whom); in my country the cellular frequencies were auctioned off for dozens of billions of euros to the cellular service providers, with strings attached, e.g. that the networks had to at least use and support 3G.

The same regulatory body (BNetzA) is also responsible for licensing telecommunications providers in general. This is probably the crunchpoint - if you're not a licensed telecommunication provider, it's simply illegal to offer telecommunication services.