Alternate phone providers

I currently have AT&T cell service. I am looking at getting a back-up "pay as you go" phone and want an alternate provider. Does anyone know which phones use other providers and does anyone have experience with those services - quality and coverage.

Thanks

Net zero is the cheapest. Seems to be on version network. Trac phone is same company. Not as cheap, but phones have more features.

A.T.&T. Go phone. Best coverage, and the cheap phone will work with your A.T.&T. regular SIM card. Then T-Mobile. Not as good as A.T.&T. but next best we have used.