I can barely hear an incoming call.

Make sure you adjust the volume level while you’re on a call. On iPhones and iPads, system sounds, music, call and ringer volumes are all controlled separately. Other troubleshooting steps below.

If you can‘t hear a person on a call or voicemail or if the sound isn‘t clear on your iPhone (Apple guide):

https://support.apple.com/en-us/HT203794

Clean the speaker grille with a swab or wooden toothpick, then go thru the troubleshooting steps in the linked article to rule out software issues. If nothing helps it might be a hardware issue.