Microphone problems with iPhone 7 and 7 Plus on iOS 11.3
Apple reports in an internal document distributed to Apple's Authorized Service Provider this week and forwarded to Some iPhone 7 and iPhone 7 Plus users may see a grayed out loudspeaker button during a call. The issue may also prevent affected customers from being heard during phone calls or FaceTime video chats.
Repairers were instructed to ask customers to first disconnect Bluetooth headsets or other audio accessories connected to their iPhone or to see if that corrects the problem. If the loudspeaker key remains greyed out during a call, the repair service providers should carry out a diagnosis. Affected devices show in the diagnostic window a message "Device could not detect Dock" or "Accessories not supported", in this case, a repair should be made
Whether the repairs are always carried out free, is not in the document However, in the past, Apple has been very accommodating in such cases.
How widespread the problem is remains unknown. Why it only occurs from iOS 11.3 and higher, Apple also did not reveal. In this case, a software update has probably caused a hardware defect.