Why can't I hear the person on the other end

Hi Tom,

By no means am I a Samsung expert, but there are two speakers in your phone; the earpiece speaker which is what is normally used when you're on a phone call with the phone up to your face, and a lower speaker that is switched to when you turn on speaker mode, as well as things like playing music or any other general listening.

So the obvious conclusion is that there's something wrong with your earpiece speaker, while your lower speaker is working fine. Exactly what the issue is may take some more diagnosis. To start with, have there been any changes to the phone recently, such as an operating system update or any kind of physical damage; e.g., being dropped, getting wet, etc.?

It is entirely possible to replace the speaker should that turn out to be the problem; unfortunately iFixit doesn't have guide specifically for that, but there's a French site called SOSav that does have one.

Earpiece speaker Galaxy S9+ repair - Free guide - SOSav

You'll definitely want to try software solutions first; do an update if one is available, and as a last resort before turning to a hardware fix you can try completely wiping and reinstalling the phone's operating system to see if that helps.