http://140.124.181.188/~khchung/cgi-bin/plutil.cgi
Use this url, convert that carrier.plist file that you have saved on your desktop, you can let it overwrite the .plist file on your desktop. Then you will be able to open the file in MsWord or Wordpad. Save it and upload to your iphone. Also, word will ask you whether you want to just save it or save it in word format. Do NOT save it in word format, just save it after editing as it is.
Now, i think you said that the allowedgeediting is already active. Then you might just have to find old att settings on the internet.
Also i suggest you try this. it sounds confusing but just go step by step and figure out your mcc/mnc code
On the iPhone start the phone app and switch to the keypad and type
*3001#12345#*
and then tap the green dial button. That opens the iPhoene app Field Test. Tap "Cell Information" The last two of the set will display something like (in your case)
N:3 #:262
meaning the MCC/MNC code is 26203.
N:10 #:234
meaning the MCC/MNC is 23410
Open the website
http://www.flatbatt.co.uk/Unlocking/...ork_codes.html
and search for the result, it will show up the related carrier. Thats the one, your iPhone "thinks" you are using.
Once you do that, check/search you carrier.plist or info.plist file(i dont remember which one) in word and make sure your mnc number is in there.
Try this and the url that is provided and give it a shot.