An internal Apple document appears to confirm another issue that has popped up with iOS 11.3 and pertains to the iPhone 7 and iPhone 7 Plus smartphones. On those devices, the update has created a microphone issue on what Apple says is a limited number of devices. Affected users may see a grayed-out speaker button during phone calls. The issue

Source: Hot Hardware – iOS 11.3 Update Crippled Microphones On Some iPhone 7 Units According To Leaked Apple Doc