At the start of the game she will answer the phone automatically. After this when the phone pops up on the side of the screen there doesn’t appear to be anything that you can do to answer it. Well turns out that there isn’t anything that you can do about it at all. The phone just pops up to tell you that you have a message, but you cant actually pick the phone up and listen to the call.
If you go into Juliets stuff in the main menu, you will be able to read the messages that popped up during the game.