Why can't I hear any sound when I make calls?

If you can’t hear any sound during a call, try turning up the volume setting using the power button on the side of the phone. If that doesn’t work, it’s probably a mechanical problem. See our repair guidefor replacing the earpiece speaker on a Nokia Lumia 830.

Before you trying repairing anything, take a look at the earpiece speaker at the top of the screen. Sometimes oil and dead skin can build up a small layer covering up the speaker grate. Try taking a Q-Tip [preferably dipped in an strong alcohol solution] and cleaning the grate. Try not to press too hard, but that might help. If not, then I'd follow Adam's advise and replace the speaker.

Sometimes it related with coverage, do you have good reception of gsm signals? Have also tried another sim? Maybe the number you tried to call isn't there anymore?