[axusgrad]

It's the network effect, AT&T and Verizon have the biggest networks, therefore the most valuable.

Apparently there are lots of pre-paid plans that do what you want using the big networks, not to mention T-mobile and Sprint with their smaller networks.

[MichaelGG]

That's not a network effect. A network effect is a growth pattern related to having other users on the network adding value to you. In this case, it wouldn't matter if I was the only AT&T customer, they'd be more valuable simply because of coverage area.