Since I upgraded to iOS 6, I've been having a major issue with iMessage. I'm unable to initiate iMessage conversations with other iMessage users. When I draft a message, my iPhone does not recognize that the recipient is an iMessage user. The message sends as an SMS, but when the recipient responds, I receive an iMessage, and then I am able to continue the conversation in iMessage. Similarly, if someone initiates an iMessage conversation with me, I am able to responds using iMessage, but if I initiate a conversation with that same person, my first message will always be sent as SMS. I've turned iMessage on and off multiple times, and confirmed that my phone is set to Send and Receive iMessages via my 10-digit phone number. I've also backed up and then restored my phone. Any ideas how to remedy this?