I suppose the question is, should you even have to do this? Can't you assume that if a phone has iMessage open, you assume that number is active and poll to se

I suppose the question is, should you even have to do this? Can't you assume that if a phone has iMessage open, you assume that number is active and poll to send keepalives. If the user has not opened iMessage for a while, you disassociate the number with iMesage. Before trying to send a message via iMessage, the server would check and reply if that user is still associated, if not the phone will fallback to SMS.