When they say carriers use VZ towers that is true, but...
Many carriers share towers. The little guys use the big boy towers exclusively. Think of it as them leasing bandwith. The problems u run into is The owner of the towers get first priority, then the next highest payer get next priority and so on.
So if you are in a conjested area VZ customers call, text and data go through first, then say straight talk, then say cricket.