There are many reasons to unlock your cell phone: to switch carriers without having to purchase a new phone, to be able to use your handset in a foreign country with one of that country's carriers (thus avoiding expensive international roaming charges), or to increase your phone's resale value on eBay. Unlocking a cell phone is very simple process, but one which most of us know little about.
A search on Google isn't always helpful, either: many of the search results I found were for unfamiliar, shady-looking websites that claim they will unlock your phone for a fee (usually around $5 -$30) but that I wouldn't feel comfortable giving any money to, let alone mailing my phone to if a firmware update were required. In most cases, your phone must be a GSM phone in order to be unlocked, and only T-Mobile and Cingular (now AT&T) offer GSM service.
The process for unlocking your T-Mobile phone couldn't be easier if you have been a customer for at least 90 days. Just email them and ask to unlock your phone. They'll ask for your headset model and some other information and email you back the unlocking instructions. The whole process took me five days (but could have taken longer, according to the original estimate they gave me), so if you're unlocking your phone for travel purposes, make sure to plan ahead. The one catch is that you'll need to have a non-T-Mobile SIM card inserted into the phone in order to unlock it. This shouldn't be too hard to obtain, though: just track down a friend who uses another carrier (one that also uses SIM cards, such as Cingular) and borrow their card for the three minutes it will take you to unlock your phone. When you take the card out and replace your original card, your phone will still work, and their card will still work, but now your phone will be useable with any carrier. T-Mobile will only honor one unlock request per 90 day period, regardless of how long you've been a customer, so if you have multiple phones to unlock, it will take a while, but it will also be free.
If you want to speed up the process, send them this information along with your initial request:
1) Carrier name listed on the handset (like T-Mobile or Cingular)
2) IMEI (obtained by dialing *#06# or by looking on the handset's information sticker, which is usually located under the battery)
3) Handset brand
4) Handset model
They will then send you your unlock code along with instructions like the ones below:
"Before you start you must have a foreign (non-T Mobile) SIM card entered into the handset. If the display reads "Enter Special Code", enter the unlock code and press "OK". If the display does not ask for Special code, please enter the instructions below.
1.Press the Menu key on the keypad, and then quickly press 073887*.You should see the Enter Security Code screen.
2.Enter 000000 and select OK.
3.Scroll to Unlock SIM select it.
4.Enter the SIM unlock code and select OK.
5.Press the End key to return to the main screen.
NOTE: If the phone displays, "Please wait to enter special code" or "Contact service provider," you will have to wait for it to change back. Please be aware that the phone must stay powered on to do this. If the battery is low, be sure to plug it in. It could take 15 minutes to an hour to change back. If the phone does not change back, the handset will need to be replaced."
That last sentence sounds a little ominous, and I'm not really sure what the risk of this happening is, so proceed with a bit of caution. I didn't have any trouble. Also, the instructions did not quite match what I had to do, but it was easy enough to figure things out based on the screen prompts my phone gave me.
If you have trouble, it may be because some phones require their firmware to be updated, which means that you might need to take your phone into a store (which is not quite as painless and might involve some attempted upselling by store employees, so go in with your guard up).
Each phone has a unique unlock code that is based on the phone's serial number (also referred to as the IMEI number) and carrier, so you won't be able to find a free unlock code online for your phone. Cell phone companies, of course, do not publicize information on unlocking their cell phones since it is in their best financial interest to have ignorant customers.
I am not an expert in unlocking cell phones, so my instructions are probably not foolproof. However, at the very least, these instructions should put you on the right path to increasing the value of your phone with no money and little effort by getting it unlocked. For more information on unlocking cell phones, check out this very informative FAQ from The Travel Insider.