depends on what phone you buy.
Sebrina, Straight Talk uses all four of the major U.S. carriers' networks; Verizon, AT&T, Sprint, and T-Mobile. What determines what network a phone or your phone will use with Straight Talk will depend on the phone model (not the service card you buy) or the SIM card used if a GSM BYOP phone.
Each phone model Straight Talk sells will have a predetermined network which it will use. Straight Talk determines what network the phone will use. A model may have two versions --for instance, one running on Verizon and another using Sprint (as is the case with the Samsung Galaxy SIII). A phone on Straight Talk service will only use ONE network and will do so no matter where you travel in the U.S. and for as long as the phone is on Straight Talk service.
Contrary to popular belief, a phone will not use one network in one area and switch to another network in other locations. If a Straight Talk phone uses Verizon, for example, it will use Verizon no matter where you are or where you travel in the U.S. It will NOT use Verizon in one city and AT&T in another. Many believe that a Straight Talk phone will use the strongest signal available no matter what the network and that is NOT true.
If you take any given phone here on the Walmart website that Straight Talk is selling, you will see that some models use Verizon, some use AT&T, some Sprint, and even some use T-Mobile.
If you bring an "unlocked GSM" phone to Straight Talk service, depending on the bands the phone can use, you will have the choice of using it on either the AT&T network or the T-Mobile network.
This $45 Unlimited service card will work with any Straight Talk phone regardless of the network it uses so you do not have to worry about that.
In my opinion, Straight Talk is a good choice for a prepaid provider. I came from Sprint contracted service to Straight Talk and haven't regretted it. In fact, I just had a problem with my phone not receiving data and I could not get it to work even after fiddling with the APN settings. I called Straight Talk and talked to two reps. The first one asked a lot of questions and then transferred me to another tech support person. The tech support guy was very patient and spent over a half-hour with me and finally figured out what I had done incorrectly. My phone is just out of warranty but still they spent almost an hour with me correcting the problem with hardly any wait at all before being connected with a rep. Yes, not what you normally read about Straight Talk's reputation for having "no service." Even though the reps are sometimes difficult to understand (support is outsourced), some of the reps are very patient and will repeat as many times as necessary if you are not understanding what they are saying. I had to have the guy repeat several settings and he kindly used "T as in Tom or A as in Adam" etc. until I understood what he was saying without getting indignant about it.