Built-In Mic Not Working

I have done a bit of a troubleshooting myself on my iPhone 7 and i found out that the built-in mic can not function at all when the speaker is working at the same time (making or receiving calls, Siri, recording a video).

But if i connect my phone with a bluetooth headset (with microphone), i can make and receive incoming calls as usual.

Need advice ASAP ! 😭