Does the issue occur with all contacts or just a select few? Can you test if it happens during Safe Mode too? To activate this, hold the Power button down and then long press on Power Off on the screen. This will change to Safe Mode so tap that to restart your phone. Please make/ receive calls during this period to serif the issue persists. If it doesn't, it indicates a third party app could be causing the issue and as such, I recommend you delete your apps one by one until you find the culprit. To exit Safe Mode, restart your phone as normal.
I've had this same problem for months. Speakerphone works fine. I finally tried a different phone app, True Phone, and as of right now it works fine with the microphone. Been 3 days with no issues, Disabled the standard "phone" app that came with the phone. I have an s10e
My S10e is currently on it's second repair for this issue. Speakerphone works fine, headphones work fine.
I had it repaired via Samsung Warranty Doorstep service with WeFix mobile technician 4 weeks ago. It was a great service to be fair, and he replaced the phone mainboard. However, over the next couple of weeks the same problem returned with nearly all callers complaining they cannot hear me.
My phone has now been sent away to a Samsung Service Centre. Not sure what else they can do, except replace the mainboard (again). Don't see what difference that will make to be honest. Maybe it is a software issue after all.
You can run a test, in the Samsung Phone app type *#0283#. Select RCV Packet Loop ON and start speaking. See if you can hear yourself back clearly. This test mode will have two options to test the normal microphone and the loudspeaker microphone. Samsung support chat can help you through this process (although beware they'll also want you to factory reset your phone for "troubleshooting")
Hoping the problem is sorted with mine, but I'm not too hopeful.