Hi,
To isolate whether the problem is software or hardware related, try starting the phone in [https://www.att.com/device-support/article/wireless/KM1320238/Samsung/SamsungSMA125U|safe mode] and check if you can be heard OK when on a call (normal calls, not loudspeaker calls).
If you can then a downloaded app is the cause of the problem. A downloaded app is one that you downloaded and installed on the phone and wasn't pre-installed when you first got it. You will have to uninstall each downloaded app, one by one and test the phone in normal mode between each uninstall. If it works OK then the last downloaded app that was uninstalled caused the problem.
if still no good, is the phone in a protective case at all?
On the bottom edge of the phone there is a small hole where the voice is passed through to the microphone used for normal calling i.e. not loudspeaker type calls and it may be being blocked by the case.
If the phone is in a case, try removing it from the case and check if you can be heard OK when on a call.
If the phone is not in a protective case, the voice inlet hole may be blocked with dust etc. Try using a vacuum cleaner to see if it can be cleared. ''Do not'' use a pin etc to try and clear it as you may damage something inside the phone.
If still no good then it may be a problem with the microphone. The phone will have to be opened and checked. Fortunately the microphone is part of the USB charge port and headphone jack board so it is relatively easy to replace the board.
Here's a [https://www.youtube.com/watch?v=TVJN4FYk2t8|video] that shows how to replace the board. Replacement boards are available online. Just search for ''Galaxy A12 charge port board'' to find suppliers that suit you best. If changing the board yourself seems too daunting, contact a reputable, professional mobile phone repair service and ask for a quote.