Samsung Captivate :: Texting - Message Says - Use The Proper 10 Digit Phone Number

I can text anyone outside my area code, but when I try it with anyone in my area code I get a message that says to use the proper 10 digit phone number. I understand what that means, and have the 10 digit numbers programed for all my contacts, but when I hit on a contact and hit the text button only the 7 digit number comes up. That works fine with voice calls, but not with texting. I've been through my phone 20 times trying to find a way to make it use a 10 digit number for texting, can't find it.