The bottom of the package has a sticker. There will be a model number and a serial number. An example of a model number is TFLG800GTMP4. Tracfone LG800G Triple Minute and the P4 means it has an AT&T SIM and associated data settings.
If it ended in P5, that would be T-Mobile. On the CDMA phones that I have purchased, there is a P, but no number after that.
On some websites such as Walmart, the model number is there with the P4.
On the Tracfone website, you go by the page's URL. When you put in your zip code and go to the phone page, you look at the URL and make sure it has market=GSM4 or market=COGSM4. If not, just edit it and hit enter to refresh. If the URL contains GSM4, then orders from that page will be AT&T SIM GSM phones.
It would be a combination of GSM5 (T-Mobile) and SPR (Sprint). You can experiment with the market= and hit enter and see what phones show up. Some possibilities are CO (Verizon), GSM4, GSM5, SPR, and combinations of two together. You can order from any page that you manage to conjure up. The zip code question in the beginning just steers you to the phones they want you to see, for their own business reasons. Sometimes, it correlates to which carriers will reliably work where you are, sometimes it's just cheaper service for them. When you get to the order form, you can put any address you want as long as it's compatible with the credit card. The only zip code that's important is the activation zip so you get a local number. They don't smile on constant roaming.
Ten digit dialing is the new normal. If you have been allowed to use seven digit dialing until now, consider yourself lucky. It is old-fashioned but you will find it around here and there. There is such a demand for phone numbers that now there can be more than one area code for the same city. So in case two people have the same seven digit number, you have to use the area code. It has nothing to do with getting a new phone company. The old phone company is simply upgrading.