I'm using a lifeproof for my iPhone 5s, and i can't hear when I'm talking on the phone?

I was talking to my dad a few mins ago and I had to keep saying, "huh" and "what did you say".

If the case is blocking the external speaker, your only option is to poke a hole in the case where the speaker is (not a good idea), or use a Bluetooth or wired headset.

Other than that, you will need to get a case that has the speaker unblocked, if the case you have now covers the speaker.