Like many other iPhone users, I upgraded to IOS 7 pretty much as soon as it came out.
There’s a subtle bug that seems to be impacting many people who upgraded early in the piece, such that iMessage has lost its internal ‘permission’ to use your mobile number as the source of iMessages transmitted over the Internet.
People aren’t necessarily picking up on this issue by themselves, because their problem isn’t actually visible to them. It is only visible to (and only a problem for) the other IOS users that they exchange messages with. It just lurks there, driving people batty, with necessarily having an obvious underlying cause.
I’ve written this post to explain the issue and how to fix it, mostly so I can tell my own friends about it without repeating myself constantly. I hope its also useful for others in the same way!