Hello Sotai,
The issue can potentially be an issue with the iPhone microphone, or it can even be an issue with the microphone source being used with the iPhone. Example: iPhone is connecting to a bluetooth source vs. using the internal microphone on the iPhone itself. This could be a bluetooth headset, bluetooth speaker, or even a car stereo if you happen to be within range and the headset, speaker or stereo happen to be powered on. The iPhone is essentially being told to broadcast the audio from iPhone to one of those devices vs. using the internal Mic.
A quick way to isolate "Bluetooth" devices that might be using your iPhone at the time of the call. Do the following
- Go to Settings - Bluetooth - Turn this OFF for right now. Now try and make or even receive a phone call and see if the issue persists. If it does, this isolates the issue to the iPhone Mic itself.
- If your iPhone supports the ability to connect the headphones that came with the iPhone. Connect those headphones to the iPhone and see if you can record audio or conduct a phone call where someone can hear you. This will further isolate if the issue is with the internal microphone.
Assuming the headset works. Your next step would be to contact Apple Support and have them run a hardware diagnostic to see if it comes back reporting any issues. They can also advise additional steps to take before getting the iPhone setup for service options.
Two of the things you should do before contacting support.