Why isn't my mic working when I call someone?

Missing a screw shouldn't cause that. The phone has 2 mics and the one your having an issue with would be the one on the bottom connected to the charge port. Its right in between the charge port and headphone jack. Make sure the flex cable coming from the charge port is secure to the logic board. If you can open it and just reseat the connections and make sure the mic is in place properly. Also look for damage around the charge port area, there are some small components that could get damaged but thats less likely. Sometimes the mesh on the bottom covering the mic can get clogged but I dont think thats your issue.

Try clean the small hall near charging port connector if not replace new charging port