I have an iphone 3G and I am occasionally having trouble sending texts to certain phones ( not sure if it is related only to certain other carriers?). For example, I can receive a text message from certain people, but when I try to reply to them, I immediately get a text message back from phone number 1121611611 which says "Error Invalid Number. Please resend using a valid 10 digit mobile number or valid short code." followed immediately by the words I tried to text. This only happens with certain people's phones; most of the time I don't have a problem receiving or sending texts.