At first, iOS 12 appeared to be a more successful update than iOS 11, but as it turns out, experience is not as flawless as expected for everyone.
According to reports, a large percentage of users iPhone has problems with the Phone application that freezes, hangs or crashes suddenly.
This technically makes it impossible to use the iPhone as a phone, with standard solutions such as restart making no difference.
Judging by online publications, including Apple's forums, the issue is not limited to an iPhone but to many models, such as iPhone X, iPhone 8 and iPhone 7. Older versions such as iPhone 6 are also affected.
Some users attempted a full reset of their device without fixing the issue for everyone. Apart from the problem with the Phone app, all the rest work properly on the affected devices.
The iOS 12.0.1 update also does not fix the problem and some users suggest that resetting network settings can resolve the issue, though it is still a matter of "luck" if it succeeds in everyone.
For now, there is no known way to deal with, and access to Apple's support services seems to be the only way to survive. OR Apple has not publicly recognized the issue and remains to see if we will soon have one patch.