I have an AT&T One X that I bought outright. I then bought an AT&T SIM from Straight Talk and use the Straight Talk service. I also later unlocked the phone and bought a T-Mobile SIM from T-Mobile just to try their service. The unlocked One X worked fine with T-Mobile. T-Mobile only offers HPSA+ service in my area, not 4G LTE, so I don't know if it would work with LTE service. But T-Mobile doesn't have good coverage in some of the rural areas where I travel, so I went back to Straight Talk on the AT&T network. Bottom-line: pop in the Straight Talk SIM, AT&T service and phone number; pop in the T-Mobile SIM, and T-Mobile service and number.
AT&T recently opened their LTE service to prepaid phones. I had to buy a new 4G LTE SIM from Straight Talk, but now for $45 per month I get unlimited calls, text, and data at 4G LTE speeds. (If you read the Straight Talk fine print, you get 2.5 Gig of data at LTE speeds, and data is throttled after that, but $45 is still a lot less than I would pay with AT&T.)