I tried to send a text message from my iPad mini to a friend who has an Android smartphone. When I put her mobile number in the send space, it turns red and says "This contact is not registered with iMessage." I then tried to send it to her email address, and received the same message. I am able to send her text messages from my iPhone 4s, but not from my iPad mini. Why is that? Is there a fix for this? Thank you.