During phone call I can only be heard in speaker phone mode.

Hi,

Use a strong light and a magnifying glass to check if the voice inlet hole, located onthe bottom edge of the phone is blocked with lint etc.

DO not use a pin or probe to try and clear it (if blocked) as you may damage the microphone which is behind the hole.

Try using a vacuum cleaner to suck out any debris that may be blocking the hole.

If this doesn’t work then the phone will have to be opened and the microphone checked and replaced.

Here’s the Ifixit Samsung Galaxy A51 Teardown guide that may help.

In the image at Step.5 hover your mouse over the 3rd image to the right and then you can see it as the main image at step.5 and the microphone is visible on the bottom to the left of the charge port on the board.

As a DIY repair it may be easiest to change charge port board (supplier example only) and check if that resolves the problem