I cannot hear caller on my iPhone 12

If not isolated to hardware (meaning that your speaker works outside of calls) it is likely a carrier related issue.

Possibly an issue with the SIM card or SIM card reader? Hard to say for sure

Can you not hear others on speakerphone or Bluetooth as well?

The call speaker is at the top by the notch

Creat a voice memo and play it back. Voice memos should have an option to play via the top speaker.

If low or no sound from that top speaker with voice memos you need a repair