Maybe you accidentally turn the volume down at some point, not even noticing. During a call, press the volume buttons on the side of your iPhone 6, and turn it up.
You will need a toothpick for this. Clean the receiver gently, and make sure to remove all dirt and dust from it.
Check if the Bluetooth is turned on. If that is the case, turn it off. To do that, simply navigate to Settings, tap on Bluetooth, and turn it off.
Press and hold the sleep/wake button, and when you notice the slider, just drag it to restart your device. It will turn off, so you will have to press the same button, to turn it back on.
Is the new iOS version available? If the answer is yes, make sure to get it in order to upgrade your device. Connect your iPhone 6 to a power source, and to the Wi-Fi network. Go to Settings > General > Software Update.