I shattered yet another iPhone digitizer so my boss hands me a Samsung Galaxy S3. The problem is that my I still have some time left on my AT&T contract and the company phones are all T-Mobile. Their is however a bright side; in addition to AT&T and T-Mobile both running on GSM networks, both the iPhone 4 and the Galaxy S3 take micro sim cards. At this point it's just a matter of unlocking the phone.
Unlocking a T-Mobile phone so that it can run on AT&T is not all that complicated. You'll simply need to aquire and unlock code and set up an access point name (APN). The unlock code allows another carriers sim card to work in the unlocked device while the APN tells the phone how to connect to a data network.
Unless you meet T-Mobiles conditions subsidy unlock you'll probably need to buy an unlock code. I bought mine from cellunlocker.net for $25. Follow the directions provided by your unlock code provider for applying the unlock code.
Once you've unlocked your phone and have confirmed you can make a phone call, you'll need to set your APN. To edit the APN go to Settings / Mobile networks / Access Point Names / + New APN. This will open an interface for applying your APN settings. My APN settings are as follows:
Name: AT&T
APN: wap.cingular
Proxy: Not Set
Port: Not Set
User name: WAP@CINGULARGPRS.COM
Password: cingular1
Server: Not Set
MMSC: <a href="http://mmsc.cingular.com">http://mmsc.cingular.com</a>
MMS proxy: wireless.cingular.com
MMS port: 80
MCC: 310
MNC: 410
Authentication type: Not Set
APN type: default,mms,supl
APN protocol: IPv4
Enable/Disable APN: APN enable
Bearer: Unspecified