A cell phone carrier has to buy their phones from the manufacturer in bulk. They order X thousands of phones. The manufacturer then makes X thousands of phones with the carrier’s logo imprinted and a few other bits of specialized hardware and/or software.
When AT&T or Verizon think there’s a large enough market, they’ll let LG know to start making some solar-powered phones for them.
For what it matters, the US is a driving market in the cell phone market, they drive the market for the tried and true technologies in cell phones. It might not be a cutting edge market in the US all the time, but it has it’s moments.