Background sound when talking to someone on iPhone 5s

Hi, every time I speak to someone on my iPhone there's this background sound making it very hard for me or the person on the other line from hearing each other. The background noise sounds like someone is pressing one of the number keys but this happens with every call I make so it can't be someone purposely pressing the keys. Ive updated the iPhone, restarted it, cleaned the receiver, but the problem still persists. Any help would be appreciated, thanks.