Apple has acknowledged the microphone issue with some iPhone 7 (Plus) models, running iOS 11.3 or later.
According to an internal distributed report, Apple Authorized Service Providers must first ask customers to disconnect or power off any Bluetooth headsets or other audio accessories connected to their iPhone to see if that alleviates the problem.
If an affected iPhone 7 or iPhone 7 Plus is no longer covered by warranty, Apple says its service providers can request an exception for this particular issue. The document does not specify if the repairs will be completed free of charge, but it seems likely given the circumstances.
It's unclear why iOS 11.3 and subsequent software versions are unintentionally disabling microphones on some iPhone 7 and iPhone 7 Plus units.