No sound during incoming and outgoing calls

Hey guys! So I wanted to share a little problem I've been having with my phone, hopefully one of you geniuses can offer a quick home fix and I won't have to take it to the store. 🙂

Whenever I receive/make calls, I cannot hear/transmit sounds. I've tried everything -- wearing earphones, putting my phone on speaker mode and even connecting it via Bluetooth to an external speaker, but I can't seem to make it work for the life of me. I've already reset the entire device and network settings and still haven't been able to make any progress. What's even weirder is that FaceTime works and music plays fine (via my Library, SoundCloud and Spotify), either when wearing headphones or when playing it out loud. Also, calls can get through and my ringtone works, so basically the problem has to do with hearing peoples' voices during the calls themselves.

Any ideas?

It's an iPhone 4S by the way and it's been updated to the latest OS X.

Any help would be greatly appreciated! Thanks in advance.