OK, slightly different problem to what other people have reported. I have an iPhone 5s, my MacBook Air shows up on the iPhone under Text Message Forwarding. I enable it on the iPhone, open the Mac, get the dialog "To send and receive messages from +44 5555 555555" on this Mac, enter the code below on your iPhone now". I enter the code, the Mac says forwarding was cancelled and it's disabled on the iPhone. I'm on the O2 network in the UK, if that makes any difference. Thanks!