iMessage on my iPhone 4S only works when I am connected to WIFI
I can only use iMessage on my AT&T iPhone 4S when I am connected to WIFI. If I attempt to send when connected over cell (HSPA+/"4G") I get a long pause while the phone tries to send the message and eventually an error message saying that the message could not be sent. At that point I can send as SMS. Any ideas how to fix this?
Thanks.
iPhone 4S, iOS 5.1.1