When I send my wife an iMessage to her cell phone number, logically it goes to her iPhone, but it doesn't show up on her iPad. When I reply to an iMessage sent from her iPad it goes to both her iPhone and iPad. How can I send her an iMessage to her iPad from my iPhone to start a conversation when I know she isn't near her phone? I don't understand why messages only go to her iPad if she starts a conversation on her iPad first. 
