i just unlocked my 8310 thru at&t
i have no idea if this is going to work for every 8310 but ill tell you what i did step by step...
1. go to "settings"
2. go to "options"
3. go to "advanced options"
4. go to "sim card"
5. when the sim card security is on (phone is locked) the screen will be blank.. type in "mepd"
6. type in "mep" hold down "alt" and press "2"
7. this is the screen where you need the unlock code. the code for mine might not be the code for your phone but i guess its worth a try it is 16 numbers "4640799680641351" and click ok. it will tell you code identified if its the correct code for your phone.
if its not the code for your phone and its an at&t phone youre unlocking try using someone's phone who has at&t. act like you are the customer (they will ask for the customers full name and last 4 of social) and just tell them you are going to travel over seas and you want to unlock your new phone and they will give you the correct code.. they will text the directions i just gave you and the code to the at&t phone you call from.
lol @ the customer service guy telling me i have to insert an at&t sim card when i unlock the phone, its not true. i had my t-mobile card in the phone when i did it =)
btw thank you to the person who posted to call at&t and tell them im traveling for the free unlock code... i would have never guessed!