When i answer my samsung galaxy A12 no one can hear me. I can hear them very well but they cannot hear me at all. Very upset about this!!

If your Samsung Galaxy A12 screen won’t turn on but it registers that it is charging, there are a few potential solutions to consider such as performing a forced restart, changing the charging cable and adapter, allowing your phone to completely drain its battery until it turns off. After that, connect it to a reliable power source and let it charge for a minimum of 30 minutes.

If the issue persists after trying these steps, it is advisable to contact Samsung support or visit an authorised service centre for further assistance and repairs.

Well, there is no specific reason behind this error. It’s suggested to remove and register your fingerprint again. 
Go to Settings 
Tap on Biometric and Security
Tap on Fingerprints
Enter Your Pin, Pattern or Password
Tap on the “Register Fingerprint” Option
Now Tap to Remove Old Fingerprint
Tap Again to Confirm “Remove”
Tap “Add Fingerprint”
Do follow the instructions on the screen to register your finger again and see if it helps.