Can't hear them/they can't hear me during phone call

I've seen a Galaxy S4 with similar problems. In that case it was a bad in-call audio chip. Perhaps an audio chip or something else in the audio circuit was damaged during the camera replacement?

If you bought this used, then maybe you need some repairs done. You would need to change the earpiece speaker and microphone (which i believe is in the charging port). If it wasn't used then it may be a software issue. You may either not have a strong signal through your carrier or you may need to try restoring your phone. Try holding down the home button and the power button at the same time for about 10 seconds until it restarts. If its your carrier, which I have seen before, try switching your sim card. I have had many customers come in with the issue being their carrier. If all of that fails, take it to apple. If this is a new device within the 1 year warranty then they will swap it free of charge. If its used, I would still take it to apple and see what the cost for a repair is or go to your closest carrier location and see if they can get you a replacement, but be warned, the replacements from most carriers are just refurbished phones. I have seen a lot of customers get a replacement sent to them that barely works, multiple times to the same person. So first try apple because they will give you a brand new device and they are usually very good with their customers.