Welcome MeliaY,
Thank you for using Apple Support Communities. Based on your post, when you receive a call on your iPhone 12 you can't hear the other person on the line. We'll do our best to assist.
If you can't hear during a call, we'd recommend: If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker
Does this happen with FaceTime calls, as well? Have you noticed this while using Bluetooth accessories (such as, headphones or AirPods)?
- Make sure the ringer/silent switch is set to ring.
- Go to Settings > Accessibility > Audio/Visual and slide the audio volume balance slider all the way to the right.
- Repeat the same steps above, but change the slider to the left side.
Listen for the device speaker. If no sound plays or the sound is distorted/muffled then you are likely experiencing a hardware issue.
Put the slider back in the middle of Audio/Visual settings above. Then test out a ringtone in Settings > Sounds & Haptics > Ringtone. Tap one to play it and listen from the receiver. If the same issue occurs, then this is likely hardware related.
In case it needs to be brought in to a local service center, get ready for the visit: Get your iPhone, iPad, or iPod touch ready for service
Select your region and search nearby here: Find Locations
Due to the current conditions around the world, some stores may have limited hours or appointment times. They also may require specific things to visit them (Such as, scheduling an appointment, wearing a mask, calling to check-in, etc.).
Hoping this helps out, take care!