Hi Santiiago77,
I apologize for not recognizing the fact that your phone's microphones works perfectly. In your first post, it indicates your iPhone is running iOS version 9.2. Just to be sure you are, could you verify that under Settings > General > Software Update please? If you're using the latest iOS version, you'll see a message that says "Your software is up to date." If you've already verified it, you can skip that step. Usually updating to the latest iOS version helps in some cases.
Once you've done that, let's try restarting your iPhone:
- Press and hold on the Sleep/Wake button until the red slider appears.
- When the red slider appears, drag it completely to turn off your iPhone.
- Once the iPhone is off, press and hold on the Sleep/Wake button until the Apple logo appears.
When you're done with that, try calling someone and checking if you're able to hear them. Then verify others can hear you when they call you. If your problem still persists, this may be a problem with iOS that can be fixed by resetting your iPhone to factory settings, but be sure to backup your iOS device first. If you need help with backing up your iPhone, and resetting your iPhone to factory settings I've included guides written by Apple below:
How to back up your devices using iCloud or iTunes - Apple Support
Erase all content and settings on your iPhone, iPad, iPod touch, or Apple Watch - Apple Support
If you have any more problems, please keep me updated and I'll continue helping you further.
🙂