iPhone 13 Mini - videocalls - speaking audio problems

When making videocalls on WhatsApp, others complain that the audio cuts out intermittently; they see my mouth moving but no sound comes through.

This has been an issue since I first bought the iPhone.

After 4 appointment, Apple have not found the problem with the microphone despite clear evidence to the contrary and I have followed all trouble shooting (reinstalling phone from factory settings, removing apps).

Any others with this problem?