Unable to be heard - iPhone 7

Read this support guide:

Get help with the microphones on your iPhone, iPad, and iPod touch - Apple Support

If you're sure that your mic works fine (for example recording a voice memo) and the issue still persists, then contact Apple Support directly: Contact Apple Support

Regards

Giulio

Well, Stu, I know that what I'm going to tell you won't be helpful, but as a matter of interest I'm using an iP7 with iOS 11.2.6 and everything is working fine.

That being said, and knowing that you have already checked your settings and read the troubleshooting steps provided in my previous link about mics, it's difficult for us to give you an appropriate answer remotely. We are just users like yourself.

I would suggest that you contact your most convenient Apple Store or Authorized Service Provider and have your device inspected. They will diagnose the issue and give you all the availble options.

Apple - Find Locations - Location Selector

Kind Regards

Giulio