Re: SHAKEN/STIR Caller ID Authentication
It's not chiefly about what they charge consumers, but what they charge each other.
Every time you call somebody on a different network, or make a long-distance call, that call gets routed through 2, 3 or more networks. Those networks have detailed records of all the calls they route on each others' behalf, and every month they'll send each other huge invoices for those services.
That's one way the networks can afford to offer flat rates to consumers: the calls the consumer actually makes are not the biggest part of revenue they get, the calls made to them are just as important. Possibly more so, if the market is suitably competitive - because the rates between companies are set by long-term, wide-scale agreements, so less elastic.
To put it another way: a lot of people say, in the context of Facebook, that "if you're not paying for the product, you are the product". Now, you've just told me you're not paying for the calls you make, only for the connection to make them on. What does that tell you about the market for phone calls?