https://arstechnica.com/gadgets/2018/12/apple-releases-a-tiny-ios-update-in-12-1-2-fixing-connectivity-bugs/

Apple released a new version of iOS yesterday, and the public notes this time apply just to iPhones. Labeled iOS 12.1.2, it arrives just two weeks after 12.1.1 hit, and primarily, it fixes a couple of bugs.

Both bugs are related to cellular connectivity and apply only to the three phones just released this year: the iPhone XR, iPhone XS, and iPhone XS Max. One is an issue with eSIM activation on those devices. (eSIMs are a new feature in these phones that allow a single phone to have multiple numbers or carriers; they're useful if you, for example, have separate personal and work phone numbers or are a frequent international traveler.)