My wife and I have an iPhone 5s each. She's currently in the UK, I'm in Dubai. Both iPhones are set up for iCloud, me under my Apple ID and hers under hers - naturally enough. Both our phones are unlocked, so when she got to the UK she bought a local SIM card and then texted me to let me know her new number. I texted her back, all is good. She then sent me some replies that came through as iMessages, not SMS. Every message I send her seems to be SMS, though - it appears in the green bubble, not the blue one. So how does this work? From what I've read, iMessage is transparent to the user, so what actually determines whether a message is sent as iMessage or SMS? How come my wife has been able to send me iMessages but all my messages to her seem to be SMS? EDIT - could the problem be related to the fact that my wife's iPhone was set up and registered with one phone number, but while she's in the UK she's running it with a different SIM card with a different number?