Questions about Cell Phones/Unlocking

So I tried a search and looked on Google and I'm having trouble finding a definite answer.

We have T-Mobile and my sister needs a new phone. We have been looking on Craigslist because it seems cheaper than eBay.

Mainly I'm wondering if it has to be an AT&T or T-Mobile (GSM) phone. I know that Sprint and Verizon don't use SIM cards, but I thought maybe the phone still had a SIM card slot. Right now my sister really wants a Palm Centro, which seems to come on everything but T-Mobile. We were kind of trying to find out if we have to get the AT&T one and unlock it, or could we use Verizon or Sprint? I mean no they don't use SIM cards but its the same phone so wouldn't there still be a slot?

Aside from this I just have questions about unlocking. First off if anybody knows any free ways to unlock phones/Centros. Secondly, when searching on Google I've seen the sites online that say they can give unlock codes for $19 or there was another for $4.99. I just wanted to know if these sites work. Also I don't want to try the cheap one and have it not work, but I also don't want to pay $19 because we're already spending enough on the phone.

Anyway sorry for the lengthyness, any help will be appreciated and rewarded with +kma. Thank you!

AT&T and T-Mobile use GSM, Verizon and Sprint use CDMA/PCS. They are not interchangeable, so for example you cannot use any GSM phone on Verizon. The phone manufacturers often make the same model in both flavors, but they are NOT the same hardware. In the early days of GSM there were some hybrid phones that worked on both platforms, but they mostly sucked and are pretty much nonexistent today.

SIM cards are only found on GSM phones too, CDMA phones will not have anything remotely similar to a SIM card, which is one of the reasons I prefer GPS -- you activate the SIM, not the phone, so you can toss your SIM card in lianyli GSM phone and it will work. Activation is no longer tied to the phone itself. GSM is also an international standard, supported my more countries and will one day likely be THE standard worldwide, unless/until something better comes along.

Regarding unlocking, if you have been with a carrier for a length of time, they will give you the unlock code for free. If you're a new customer, then they usually won't. With GSM carriers, a lot of people will simply tell them that they're planning to travel overseas and will be buying a local prepaid SIM card, so they need an unlock code.

Depending on the phone, there are often ways to unlock them yourself posted online, at least in the old days. I think locking techniques have become more advanced though because I seldom see unlocking info freely available on newer phones. It usually takes special hardware or inside knowledge, possibly IMEI-specific codes, hence the scarcity of free methods.

Like dmo says, you need to get a T-mobile or AT&T phone (or an unbranded and unlocked GSM phone) if you are with a GSM carrier.

At least the older nokias were easily unlocked with a free calculator available online but the latest ones probably aren't that easy to unlock. Unlocking is more and more restricted nowadays and you'll most likely have to pay someone to do it for you. I am guessing they're really careful now not to leak any unlocking methods after we all had a free ride with the Nokias. I've had a few phones unlocked through ebay recently, and it was pretty painless and cheap. In any case, you can google in advance for the model you want and see what unlocking options are available to you.

The problem in your case is that to get the centro from AT&T for $99, you'd have to get a 2 year plan with them. If you want to use it unlocked on t-mobile, this is obviously not going to work because now you'd have a monthly bill from both TMO and AT&T. You'd be better off getting an unlocked one from Palm for $300.

Well the thing is, we see one on Craigslist for $180 we want to buy. It's from AT&T but I guess it would need unlocked before her sim card would work. So thats the only issue really, then pay for an unlock code and hope it works. Plus would we go with the $4.99 or the $19 one? It seems they do the same thing but it sounds fishy. I'm sure I could figure out a cheap way to unlock it I guess.

Anyway as of today we are seeing a used 4gb iPhone on Craigslist for $175 so now shes considering that. I want one too but I was gonna wait until after the 3G's come out and see if the old ones drop in price at all, plus I need at least 8gb. But its perfect for her, right now she has a 4gb nano thats not even full, and she could sell that for about $100 and just use the iPhone. The only thing that worries me about that is that shes only 13 and an iPhone seems kind of bulky and excessive for her, though for $75 of her own money I guess it would be fine and its her decision.

Would you guys suggest the old iPhone over the Centro? Also, the iPhone would need to be unlocked too, but I'm thinking I could do that myself for free because it seems like the internet has a few tutorials on this. Has anyone on here unlocked an iPhone themselves? And does anyone know if there's gonna be issues with unlocked iPhones and updating to iPhone 2.0? Maybe that would brick an unlocked iPhone out? Guess we'd have to wait til Friday to find out.