To reemphasize, because I took out/ disposed of the original SIM card, I am struggling to find ways to access the IMEI number. Is my best bet to get the IMEI number to request help at the Genius Bar? I have a newer iPhone that I use. Is it possible/safe to try and synch the old phone to iTunes to get the IMEI number?
Every GSM phone technically requires the IMEI to be available somewhere on the physical phone itself. In the case of the 3G,
it's on the SIM tray (so long as you hadn't lost the original and replaced it with an aftermarket).
Right now, you only have an AT&T carrier locked phone that you've done a factory reset to.
You will need to still have an active AT&T account to actually get the phone carrier unlocked, and cannot use any GSM carrier other than AT&T or an AT&T MVNO until that point on the carrier locked phone, and technically, any AT&T MVNO SIM card placed into the phone will only be able to do straight calling and SMS text messaging, no MMS or moble data services until the phone
is carrier unlocked so-as to permit changing the network gateways for both services. To do the carrier unlocking, you will need to have
some sort of SIM card available. It doesn't have to be your original AT&T SIM, but it does have to have some sort of GSM SIM card in it (AT&T or T-Mobile or respective MVNOs). The good news is, the SIM doesn't have to be attached to an active account. Even an older deactivated SIM or a new never-activated SIM will work. If you need another SIM card for the unlocking process, hit up Ebay for a Spot Mobile SIM as you can usually buy one for a buck or less with free shipping.