Keep your voice down while on the phone
Say you have an important call that you can’t reject while riding the subway. People can understand that, and they won’t ask you to shut your phone down for their sake. However, if you’re speaking obnoxiously loud and guffawing like a buffoon, the entire train is going to give you the stink-eye.
Basically, if you receive a phone call in public, keep your voice down at all times. The last thing you want to do is draw unwanted attention to yourself, especially if you’re the one who’s at fault. Once again, if you send text messages instead of answering a call in public, you should do so.