Sounds simple but.
Get some blu tac tac press firmly over the ear piece then take it off.
This piece easily becomes clogged with dirt and dust.
You will probably notice the dirt on the blu tack
Do this a few times and try making a call again.
I made a profile to come back and say thank you specifically to this reply. The blu tak, or 'fun tak' by Loctite I found at TruValue worked perfectly. I started slow but ended up mashing a fresh tiny piece in 30 or so times, every press you could see a few more holes clear out until I had all but one clear. (I would estimate there are 45 holes in that ear speaker screen). I started my phone back up and had to hold it away from my ear when it was on max volume. Worked perfectly, this should be top solution. Press it in there hard many times, it works!!! The 'fun tak' cost 2.99
I can`t thank you enough for this! You helped me to save some money and I was ready to buy a new phone because of this. I like my S8 and I am very happy that I can use it again. Thank you, thank you, thank you!
I'm not sure about all the technical things everyone is putting out there because I tried them and they had no affect on the problem. I simply took a baby wipe and can of air for my PC keyboard and scrubbed the ear speaker and it fixed my problem. I'd imagine dust, dirt, and pocket lint gets in there blocking sound. Hope this helps you as it helped me.