Error Invalid Number, re-send using a 10 digit number when texting a certain number, but when I put the area code in it still sends to the 7 digit number

About a month ago I added a friends number without the area code. I was in an area that I couldn't send texts from. Two days ago he texted me and I tried to reply, but when I did I got the error message "Error Invalid Number. Please re-send using a valid 10 digit mobile number or valid short code."

So, I put in the area code in front of the number and when I sent it, the phone dropped the area code and just sent it to the 7 digit number. I went ahead and deleted all of his contact information and text messages to him and re-entered the with the area code and saved him as a new contact. I tried to text the number and it still sent it to only the 7 digit number!!!

I have tried everything there is to try, but there seems to be a part of the phone that is remembering this number even after I've deleted it and everything associated with it.

Does anybody have a solution for this?? I have called Apple and they told me they have never heard of this problem before.