The cause isn't clear, although reboots and other typical troubleshooting steps don't appear to address it. There's also no indication of the scale of the problem, although it doesn't appear to be common.

Apple told Engadget that it's "looking into these reports."

The call flaw probably won't put much of a dent in iPhone X sales. It's not what you want to see in a premium smartphone, however. It also suggests that Apple's rumored emphasis on stability in iOS 12 would be coming at a good time. While this isn't necessarily a software problem, there has been a growing push for Apple to work on reliability after months of glitches.