It looks like a never-ending process has begun with the first iOS 11 bug, because with little exaggeration almost every day comes a new problem, which then spills over the Internet.
There have been a lot more here, but the latest exception is not for iPhone X users, but for the slightly less popular, yet slimy iPhone 8 and 8 Plus owners, but the current problem happens rarely for iPhone 7 as well.
Let's go over the error, which is probably software, as there was no complaint in previous iOS. KeithJ80, if you do not speak directly to the microphone during a phone call, the other side will either hear nothing or drastically blur the stream.
Apple has not yet responded to the bug, but they are using crossed fingers to release iOS 11.2 as soon as possible, and hopefully comes the bugfix.