Since the release of iOS 11.3, many users have reported a serious problem on their iPhone which disables the device’ microphone resulting in no audio while on a call.
Unfortunately, even after acknowledging the problem in iOS 11.3, Apple still hasn’t fixed the microphone issue, and it continues to affect even more users with the newest iOS version (11.4.1).
iPhone users who weren’t affected by the microphone issue back when iOS 11.3 released, have now met the bug after installing iOS 11.4.1 on their iPhone.
If your iPhone is affected by this issue, there’s currently no fix or workaround to get in-call audio back on your device. If you’ve warranty left, you can easily get a replacement for your iPhone through the Apple Service Center.
If you don’t have a warranty, the Apple support representative might deny a free replacement for your iPhone. But you have to tell the rep that it was an official software update that damaged the phone, so Apple should take responsibility for it and provide a free replacement even out of warranty.