AT&T is shutting down its 3G network on Feb. 22, something that should only affect people with very old devices, mostly flip phones. But a subset of unlocked smartphone owners are getting caught up in this mess, and some of them don't even know it.
The folks getting a nasty surprise are people who bought smartphones not designed for US networks. Some of them aren't sold here, like Huawei and Xiaomi models. Those were always living on borrowed time. The really tricky ones, though, are Samsung models that look exactly like phones sold and supported in the US, but aren't.
The shutoff will also affect people on other carrier brands that rely on the AT&T network, such as Cricket, H2O Mobile, and some Red Pocket plans.
All iPhones of the iPhone 6 generation or later will work fine after the 3G shutoff. To check the model of an Android phone and see if it's on AT&T's approved list, go to Settings > About Phone. AT&T has a frequently updated list of 4G supported phones here.
You're Blacklisted Now
Some back story: All three major carriers are decommissioning their 15-to-25-year-old 2G and 3G networks to make more room for 4G and 5G, which use limited airwaves far more efficiently. Most phones sold in the US since 2014 support VoLTE, or voice over 4G, for phone calls.
But the 2G and 3G system AT&T and T-Mobile used has a requirement that all technically compatible phones be allowed to make voice calls ... and 4G does not. So the carriers can now whitelist phone models depending on whether or not they want to support them. AT&T has gone down the whitelisting path, while T-Mobile has taken a more open approach. Verizon has always whitelisted, because it used a different 3G system that allowed whitelisting.
Samsung makes several different variants of each of its phones, for different countries. Carriers in the US sell the "U" model; Samsung.com and Best Buy sell the unlocked "U1" model. Both have been approved for 4G calling use by all the US carriers.
But several Amazon sellers, as well as B&H Photo, sell other international models to US buyers—models ending with "B" "F" or "M", for instance. Those models haven't been verified by the US carriers and may not be supported.
Searching Amazon right now for the popular Samsung Galaxy A32 5G, the top result for an unlocked unit is the "W" model, which will not be supported by AT&T. You have to scroll down to find the properly supported "U" model.
Your unsupported phone will still work with Wi-Fi data, and if you get 4G or 5G data now, you will still get it after the 3G shutdown. In that case, AT&T's restriction will only affect voice calls. If you really want to hack your way around that, you could use WhatsApp, Skype or another data-reliant app to make your voice calls.
If you have an unsupported phone and call AT&T, it'll offer you some sort of free phone as a replacement. Or, your existing phone may work just fine on T-Mobile, if that carrier's coverage works out for you; get a "test drive" SIM and see if it works.