AT&T Unlock iPhone 4 without a Sim Card installed question

Question

I just put in my request to unlock my iPhone 4 from AT&T.
My contract has expired, and my old phone number is no longer active as of about 2 weeks ago, I also no longer live in the US (I've already travelled abroad), but they said that it would have no problem to unlock my iPhone as it's all done via iTunes.
I still have my old AT&T Sim card, but the number is not activate and like I said I'm in another country.
I have heard for the unlock process to be completed you need an activate sim card installed in the phone. Does it matter what carrier's sim I use (can I use a local one?) or does it absolutely have to be an AT&T sim?
I'm in Dubai now and have an active prepaid du sim card (they said it can easily be cut to micro-sim).
Contacing AT&T from here is a nightmare (the "international" number they gave me won't dial out and the website won't let me access my old account info since it's closed now and thus can't access their web chat either).

Answer 1

From another poster in the Communities: 
Stipulations      
•     Phone must be off contract or you must be in the military traveling abroad.     
•     Account must be in good standing for the last 30 days.     
•     You must have valid email or phone number where you can be reached when the unlock is completed by Apple.  
How it works:      
1.     Call AT&T at 1-800-331-0500 and go through the prompts to get to a customer service agent     
2.     Have your IMEI # ready along with your active email address and current phone number     
3.     If your phone qualifies then AT&T will submit a request to Apple to authorize the unlock on your phone. (AT&T will tell you right then on the phone if it can be done )     
4.     Make sure you get your case number in the event something doesn't go through so you can get support from AT&T.     
5.     Once you recieve the email from Apple, open iTunes and transfer purchases, backup your phone and then perform a restore.     
6.     Once this process is complete your phone should now be unlocked to use other carriers.
It will be up to you to find a way to call a US 800 number from outside the country; do a web search and you should find suggestions.
Regards.