Using your voice on stage requires a different kind of technique than using your voice in general, or while acting in front of a camera. You will have to be understandable for the entire audience, including the people right at the back.
Hold the phone – don’t scare the audience away though! Even when you must speak louder than normal, it shouldn’t sound like you’re shouting. Apart from the fact that it would sound terrible, you wouldn’t want to end your acting career before its got started by wrecking your throat, would you? Also, to make the play believable, it should sound like you are speaking at normal volume, even when you’re not.