Ever since I've gotten the iPhone I haven't been able to send text messages to people who are not on AT&T/Cingular, but they can send text messages to me. Once I receive a text message from them then I can reply and it will get to them, but if I try to go to contacts and then text them, I get an error message saying its not a valid 10 digit number. I've tried both just their number, their number and area code, and putting one before their area code and number, all to no avail. It would be very nice to be able to text my family again who are on T-Mobile. Thanks in advance for any help!