As a rule, the higher the floor you are on, the quieter you should be. The quiet study room is on the third floor for a reason. If you have to answer your phone, be polite and go near the main staircase or somewhere else away from people who are hard at work. Whether you’re chatting on the phone or with a study buddy, keep your voice at an appropriate volume.
The same goes for headphones: if people other than you can hear your music, it’s too loud. Take them off and hold them at arm’s length to check. If you can hear them at that distance, turn it down.
On the flip side, don’t be afraid to say something if someone is being particularly obnoxious or disruptive. Give it 5 minutes, and if they are still being bothersome, politely tell them they are being too loud.