Straight Talk does not give a choice of service providers but does offer service on ATT, TMobile, and Verizon. By buying at Walmart you can see which service provider the phone is on by looking at the package. Buying on their website you can see by the model code on the phone. My wife has a Straight Talk plan and purchased a Blackberry Curve that runs on the Verizon network - voice, text, and data. In this past year, Straight Talk has stopped selling sim cards for ATT and is only selling them for TMobile service. But ST still is subcontracting with ATT and one needs the correct Sim Card for ATT from ST to make any GSM phone work on ATT through Straight Talk.
If you are on ST TMobile you do much better going to Walmart for a TMobile phone on the Walmart only TMobile $30/month plan of 5 gb data, unlimited text and 100 mins talk.
In our travels when my wife formerly had a ST plan with ATT service, we had either equal coverage where we were (with me on TMobile) or I had coverage and she didn't on ATT. Now with her ST Verizon coverage she has the better coverage in some areas over my TMobile coverage.