The iPhone XS, XS Max, and XR are the first iPhones to have dual SIM slots, which just became usable with the iOS 12.1 update. The thing is, the second SIM slot is still pretty useless in the U.S.
While the new iPhones use a traditional SIM slot for the primary SIM card (as they always have), the secondary slot uses the much newer eSIM. And while the option to set up your second carrier is already present in iOS 12.1, all the major U.S. carriers have pushed back support for this new tech.
Apparently, AT&T is pushing back support for eSIM because of incompatibility with visual voicemail. Verizon is delaying support because data on its network drops to an unusable 2G connection when used with the new iPhones. T-Mobile told PCMag that it’s still working to add support. All three are said to have it ready to roll by year’s end, though.
Sprint isn’t even on Apple’s compatibility list in the first place, but the carrier says it’s working to bring eSIM support at some point in the future.
The good news is that if you use the second SIM slot for an international carrier, it may already work. You can hit up Apple’s support page for full compatibility info.