If the call volume is normal on speakerphone and wired/Bluetooth headphones, then I'd suspect a problem with the earpiece speaker. How new is the phone? If it's pretty new, bring it back to the store and ask for an exchange. If it's more than a month old, contact Samsung about warranty repair.
Does the problem persist in Safe Mode? https://support.t-mobile.com/docs/DOC-28008
So all calls have a problem?
Just to get the simplest solution out of the way, have you tried upping the volume DURING a call? You may have already tried it, but forgot to mention it.
If it's already maxed, there's only two possible problems left. Your phones speaker is problematic, or your hearing is problematic. I'd like to lean towards the phone problem for now.