Some owners of the iPhone 7 and 7 Plus have had an issue with their handset and the microphone, this appears to be since iOS 11.3.
The issue apparently shows greyed out speaker button during calls and it can stop people being heard whilst they are making a phone call, the problem appears to be with the microphone on the handset.
Apple have told their technicians no ask their customer to disconnect bluetooth headsets and any accessories connected to the iPhone to see if this fixes the issue. If this does not resolve the problem then your handset may have a hardware issue which has been highlighted with iOS 11.3 and above.
Apple will apparently repair your device if you have this issue, although it is not clear as yet on whether they will charge you a fee for this is your device is out of warranty.
If you have had this problem on your iPhone 7 or iPhone 7 Plus then you need to go to Contact Apple Support about the issue.