imessage seems to be great, but somehow there are 2 or 3 of my contacts who don't receive my imessage texts to them, unless they are on wifi - they dont go through as texts. How do I change this? When I send them a text I want them to get it instantly, whereever they are! Imessage is a great option when we are both in wifi range, but shouldnt it just convert to text when we aren't?? Thanks