Unlocking falls into the category of cracking/hacking and is not allowed on these forums for more reasons than one.
To start with, if the phone happens to be on a contract and you so choose to unlock it; you are committing breach of a legally binding document by doing so.
If the phone is not on a contract, but happens to be running a custom version of an Android ROM such as the ones T-Mobile frequently puts on their devices, unlocking it yourself falls into the category of copyright infringement as you are modifying copyrighted software. Same goes for Apple software and the iPhone.
Furthermore, we can not prove the device ownership, and will not help commit theft.
Because of the reasons outlined above, any post requesting a network unlock will be removed.
The following are the six principles and rules laid out by the FCC for American cell phone carriers, in regards to network locks and unlocks.
Disclosure: Each carrier will have to explain its policy on unlocking clearly and concisely on its website.
Prepaid Unlocking Policy: Carriers will unlock prepaid mobile devices upon request, no later than one year after the initial activation.
Postpaid Unlocking Policy: Carriers will unlock or provide necessary information to unlock their devices to customers and former customers after the completion of a service contract, device financing plan or the payment of an early termination fee.
Notice: Carriers that lock devices have to notify customers that they are eligible to unlock their devices when eligibility occurs or will automatically unlock devices remotely when they are eligible.
Response Time: Carriers will unlock eligible devices within two business days of receiving a request.
Deployed Personnel Unlocking Policy: Carriers will unlock mobile devices for deployed military personnel for customers in good standing.
If you happen to own a handset that is not on a contract, and is running a stock Android ROM from Google like the Nexus and wish to unlock it; your options are as follows.
1. Call the carrier it is locked to and request an unlock code. If the phone is not on a contract they will supply this code to you. In order to use the code, insert a non-network SIM card (ie, the phone is locked to AT&T - insert a T-Mobile SIM), turn the phone on. It will read "restricted" or "SIM network locked" across the top. Enter the code supplied by the carrier and hit enter. It will now be unlocked.
2. Unlock it yourself at a software level. Remember that this is ONLY LEGAL if the phone is NOT on a contract and is running a STOCK version of the Android ROM from Google. Also, the slightest mistake during this procedure could result in turning your phone into a paperweight with a battery. If you are legally allowed to do this, and you are willing to absolve that risk, request help at the XDA forums here: http://forum.xda-developers.com/ This is considered cracking/hacking so I will not supply more help than that link, as discussing it further would be against the rules of Tom's.
-------2a. For iPhone users, the legality issues are the exact same. While you can use software tools to unlock your phone, it's only legal if you own the device, and are not on a contract. You can also order the iPhone direct from Apple without a lock to begin with. If however you happen to have purchased a phone from a carrier and are not on a contract, and you would like to unlock it, please seek the help doing so here: http://forum.iphone-developers.com/