Apple just stopped signing both iOS 12.1.2 and iOS 12.1.1. Meaning, it is no longer possible to download to an earlier version of iOS 12.1.3, version that Apple is currently signing. Apple stops signing older versions of iOS to force users to update their devices to more current versions.
With this move, Apple is focused on improving both the security and the stability of iOS in general, since all the new versions, introduce improvements in both directions. In addition, and although the jailbreak is not so fashionable, prevents the community can continue to develop exploits for older versions of iOS, this community is most often seen more affected by these movements of Apple.
iOS 12.1.2 was officially launched in December to correct the various errors that were presented when making use of the eSIM, but in addition, Apple took the opportunity to solve the patent problem that had been presented with the demand of Qualcomm in China.
iOS 12.1.3 was released in its final version two weeks ago, with stability improvements in both Messages and CarPlay, and is currently the only stable public version that Apple offers to all users who are forced to restore their devices in case that they have a problem in their device that forces them to do so.
Currently, Apple is testing among developers and users of the public beta, iOS 12.2, the next big update of iOS 12, which will come with a large number of new features such as new Animoji characters, TV features compatible with HomeKit, support for Apple News in Canada and much more. But while the final version of iOS 12.2 is released, it is more than likely that Apple releases iOS 12.1.4 to restore the functionality of FaceTime group calls.