If the Bluetooth is turned on, it may be the reason why there is no sound during a call. To turn it off, go to Settings, and choose Bluetooth.
While you are talking on the phone, press the volume buttons which are located on the side of your phone. Maybe at some point, you turn the volume down, by mistake.
You can clean the receiver with a small needle, or a toothpick. Remove debris, dust and dirt, but to this slowly and gently.
Press and hold the sleep/wake button, and slide to turn off. When the phone is off, wait a few seconds, and turn it on, by pressing the same button in the same manner.
When you have any kind of strange issue with your device, such this one is, check if there is a new iOS version available. Each version fixes the bugs and adds some new features, which is why you should update your device.