I have an iPhone 4s. I can send an iMessage to other iPhone users as long as they are verizon customers. I have one friend who is an iPhone 4s user as well but she has ATT. We used to be able to iMessage each other with no problems. Now all of a sudden if I send her an iMessage it will only go through as "Sent as Text Message". But if she sends me a message it goes through as an iMessage. If I respond right away it will go through as an iMessage. So basically when I initiate the message it will only send as a text message. And then she does not always get the text message. But this really just started a few days ago! please help.
The other user needs to have a device that is iOS 5 capable have iMessage and a data connection enabled for example if they have an iPhone 3 (not the 3GS ) which will not support iOS 5 or higher then iMessage is not an option on that iPhone