From what I understand… there is a big difference between the “pay as you go” phones you buy in store and the contracted plans through the carriers. The Pay as you go plans run off a specific carrier’s towers… whereas the actual contractual plan allows you to run off of their towers as WELL as towers of other companies that the carrier contracts with. So… I have my plan with Verizon… if there are no VZ towers, but there are ATT towers, my calls will still go through with no roaming charges. With pay as you go… you would have to pay roaming charges to get your calls through.
So.. the pay as you go is great if you live in the big city where there are carrier towers every couple blocks. But… live out in the country or don’t stick to major highways, etc… it can become a problem or costly.