We began with using the device ID but of course we've had to change given that's deprecated - http://stackoverflow.com/questions/6993325/uidevice-uniqueid... - iOS6 seems to have perfect answers here - we're not interested in IDs you can share between apps - this is for our app only.
If that's the case, could it be possible to circumvent that through spoofing of the number and subsequent reception of the SMS message?