Why can I only hear on the phone if I use the speaker option?

Do you mean that your voice cannot be heard by the correspondent?

Or do you mean that if you don't use the speaker option you cannot hear your correspondent?

Not using the speaker option, the only sound (voice) can come from the earpiece.

Make sure that nothing (cover or screen protector) interferes with your earpiece.

Shouldn't be this case, check your sound balance in Settings>Accessibility>Audio/Visual... it should be in the middle.

If the issue still persists, since your iPhone is still covered by warranty, I suggest that you make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector. After checking it out, they'll give you all the available options.

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.

Most likely, the speaker in the earpiece has stopped working. Contact Apple to have it evaluated:

Genius Bar Reservation and Apple Support Options - Apple

My pleasure :)

Giulio