iPhone requires speakerphone to hear calls

You take the phone to Apple and have the earpiece speaker repaired/replaced.

Apple Service and Repair for iPhone - Apple Support

FYI when you’re actually communicating via the “non-speaker” earphone …

… the physical “up” and “down” buttons (on the left side of your phone) control the earpiece volume.

(When you’re conmunicating via the “speaker” they control the speaker volume …

… and when you’re not using the phone (aka you’re “on hook”) they control the “ringer” volume)

Klg6 ~ As your iPhone 13 could be 5 years old, it's worth checking for a weak, aging battery (which can cause various audio issues.) So...

Does your iPhone 13 still have its original battery?

And what's the Maximum Capacity % here?...

Settings > Battery > Battery Health & Charging

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Grant Bennet-Alder wrote:

There is only a built-in speaker, whether you press your ear against it next to your head or turn up the volume and lay it on the table.

Actually, there are two. One is next to where you hold your phone to your ear (okay, "earpiece" is an anachronism), and one is at the bottom of the phone. If the one at the top goes bad, you can only hear the call if you put it on speakerphone so that the bottom one comes into play. I've had this happen.

I don't understand.

There is only a built-in speaker, whether you press your ear against it next to your head or turn up the volume and lay it on the table.

¿How did you expect audio to be delivered to you?

¿Are you using some sort of external speakerphone? if so, how is it connected to you iPhone?

Grant Bennet-Alder wrote:

Thanks for that!

I stand corrected. I never realized there were two.

Your initial advice to get the iPhone checked now makes Perfect sense.

If it hadn't happened to me, I might not have figured it out.

<< Actually, there are two. >>

Thanks for that!

I stand corrected. I never realized there were two.

Your initial advice to get the iPhone checked now makes Perfect sense.