12 mini speaker not working while calling

Hi there,

Try this

If people can't hear you on phone or FaceTime calls

1. Open the Voice Memos app. 
2. Tap the Record button, speak at the bottom of your phone, then stop the recording. 
3. When you play back the memo, you should hear your voice clearly.

If you still sound unclear during calls, recordings, or apps, contact Apple Support to discuss service options.

See this link for further reference

If the microphones on your iPhone aren't working - Apple Support