Phone volume

To make sure, that this is not hardware related, try a force restart first, you will not lose data:

Force restart iPhone - Apple Support

The next step would be setting it back to factory settings, without using a backup after that and testing it again.

Restore your iPhone, iPad or iPod touch to factory settings

If you create an encrypted backup in iTunes before you start, even passwords for accounts and Wi-Fi will be included in that backup.

About encrypted backups on your iPhone, iPad, or iPod touch - Apple Support

Is a case or a display protection installed, that may cover part of the speaker on top of the phone?

Did you try to increase the volume during a call? Increasing the volume without making a call will not change the volume for phone calls.