Monopolies are not only formed of one company, they can be a collusion of companies so long as the barrier to entering the market and competing with the colluders is impossibly high.
Considering the barrier to entry to becoming a cellular operator is owning spectrum, that is one of the most impossibly high barriers to usurp. Nobody has to sell you any, the owners can set whatever price they want, and there are no mechanisms to mitigate this state-created impossible to overcome monopoly on light.
Landline access, by comparison, is macro-harder and micro-easier. It is more likely you can get easement rights to run fiber on a given street than to get the right to use wavelengths of EM, but trying to get easement in an entire city can be similarly impossible.
Considering the barrier to entry to becoming a cellular operator is owning spectrum, that is one of the most impossibly high barriers to usurp. Nobody has to sell you any, the owners can set whatever price they want, and there are no mechanisms to mitigate this state-created impossible to overcome monopoly on light.
Landline access, by comparison, is macro-harder and micro-easier. It is more likely you can get easement rights to run fiber on a given street than to get the right to use wavelengths of EM, but trying to get easement in an entire city can be similarly impossible.