I have an iPhone 5 on straight talk for the past 2 years with trackphone which uses the Verizon towers. If you have a GSM phone and had been unlocked, you can get a sim and use straight talk on the AT&T, the CMDA phones can not use a sim for Verizon towers unless the new iPhone or other makes are bought to be used on straight talk. I get unlimited talk, unlimited text, unlimited dataset 3G. After 3 gs, you are throttled for the remainder of the 30 days. With the 5 s phone on AT&T you can connect with 4g lte but not with a cmda phone at this time on straight talk. My wife still uses Verizon, and my phone works where hers does but her data is 4g LTE, so it is much faster.
Last Christmas, son gave me an iPad air and put a sim in it for t-mobile and I get free 200 mb every 30 days and can buy extra gigs for $10 per gig. T-mobile does not give the coverage like Verizon, but in the larger cities I do get 4g LTE data.
This works well for me, but might no for heavy users.