Can't be heard on calls

What model phone do you have? On newer phones there is a noise cancellation feature that utilizes a mic on the back of the phone, a tiny hole near the camera. If this is covered it can make your voice weak or garbled. So take a look and make sure you don't have a case covering it, and, if you have a flip open folio case don't fold it all the way back when on a call. It's only phone calls that use this back mic (and video recordings), so it won't affect Siri, voice memos, FaceTime, voice texts, etc.

The noise cancellation mic on the 4S is on the top, a location where many cases might cover it.

Make sure during the phone calls that your microphone is not muted, for that, tap in the button that says “mute.”