iMessage
My sister just recently got an iphone 5s and I have a 4s.When she texts me it comes across as an iMessage but when I text her back it only sends as a normal text message. It won't let me send an iMessage to her. When I text other people who also have iPhones there is no problem and I can send iMessages to everyone else but her.
Any ideas?
iPhone 4S, iOS 7.0.4