Google Pixel 4a mic issue - mic won't work when earphones are inserted into headphone jack

When I am on a call, the mic on my Google Pixel 4a works perfectly. When I insert earphones into the headphone jack during a call, the mic suddenly stops working and the other person can't hear me anymore. When I remove the earphones, they can hear me again. What could be causing this? All I want is to be able to hear the person through my earphones and for them to hear me.