The iPhone XS, XS Max and XR are the first iPhones to have two SIM slots available with the iOS 12.1 update. The problem is that the second SIM slot in the US is still pretty useless.
While the new iPhones use a traditional SIM slot for the primary SIM card (as always), the secondary slot uses the much newer eSIM. And while the option to set up your second carrier already exists in iOS 1
Obviously, AT & T is rejecting support for eSIM because of its incompatibility with visual voicemail. Verizon is delaying support because the data in its network falls on an unusable 2G connection when using the new iPhones. T-Mobile told PCMag that it was still working to add support. All three should finish by the end of the year.
Sprint is not listed on Apple's Compatibility List at all, but the carrier says it is working to provide eSIM support at some point in the future.
The good news is that the second SIM slot for an international mobile service provider may already be working. See Apple's support page for complete compatibility information.
RELATED: What is an eSIM and how does it differ from a sim card?
Sources: The Verge, PCMag, MacRumors