What phone carriers are available in the USA that are compatible with a sim900 breakout board
SIM900 should be compatible with both AT&amp;T and T-Mobile. T-Mobile uses 1900Mhz for 2G and AT&amp;T uses 850Mhz and 1900Mhz. Although both carriers are upgrading to 4G, they still support 2G devices. My personal experience is that most quadband 2G devices still work with AT&amp;T and T-Mobile and there is no technical reason why SIM900 shouldn't work. However, there can be issues with incompatible data plans that are compatible only with their own devices. You need to be careful when selecting the contract and buying the sim card.

Nextel and Ting uses Sprint's network which is CDMA and thus wouldn't work. However Vodafone seems to be using T-Mobile's network and may work.