S8+ low call volume through earpiece

During the call, you should able to adjust the volume higher with the volume buttons or by going to Settings > Sounds and Vibration > Volume, and making sure all the sliders are to the right.

If that's not working, something else is wrong. Try removing the case or cover on the phone, if there is one. Restart the phone. Try playing music or videos to see if the sound quality is poor there as well. Put the phone on speakerphone or Bluetooth. If the call volume is better, that usually means if it's the earpiece that's the issue.

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@Ldoktx1 wrote:
Thanks for your answer. This phone has had this problem since I got it. I
have tried everything as far as software goes and volume booster apps boost
it everywhere but the earpiece. Again, thanks for the reply. I will see if
AT&T will warranty it. It's worth a shot.

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Your warranty is for 1 year from date of purchase.   AT&T is allowed to exchange for you for that 1 year, but your warranty is with Samsung.