This is an excerpt that I wrote for ebaY on mobile phone

Carrier Specific vs. Unlocked Mobile Phones

Many mobile phones (and PDA and smartphone devices) which are for

sale on eBay are phones which were purchased through North American carriers. The largest North American Carriers are Cingular, T-Mobile, Verizon,

Sprint Nextel, and Rogers wireless (Canada). In NA, these carriers work with the handset manufacturers to “lock” the devices so that they only work on

each specific carrier network. This practice of restricting the phone provides some security to the carriers, and they sell the phones at a subsidized

price to the consumers figuring they make more money by locking consumers into long service contracts.

A “locked” device

normally will only work on it’s original network, so Cingular phones will only work on Cingular networks for example. However, some devices can work

on other carrier networks – all you have to do is call the carrier that you want and ask them to run the ESN – the Electronic Serial Number – found on

the back of every phone, normally under the battery. This number is transmitted every time you make a phone call, it’s how the carriers know who you

are and how to bill you. The carrier can tell you, before you purchase the phone on eBay, if you’ll be able to use the phone on their network.

Some phones will actually work on multiple networks. These phones have been reprogrammed and are referred to “unlocked” phones. They are normally

more expensive because they can be used without a service plan and are portable between the different carriers. Many of the phones available in Europe

are unlocked.

Finally, and to add to the confusion, some locked phones can be used on different carrier networks. I’ve popped in SIM cards

into “locked” devices and seen them work forever on an alternate carrier, and some work for 24 hours and then can’t register any longer. The only

real way to tell is to ask the carrier to check on the ESN.

Dual-band and Tri-band and Quad-band phones

The GSM networks

in North America are different from the ones in Europe.  Most of the world uses GSM-900 and GSM-1800, while the North America and most South

American countries use GSM-850 and GSM-1900. These numbers refer to the radio frequency that the devices transmit on. So, your fancy-pants new

phone that you just bought on eBay may not work in Europe or even on vacation in the Carribean, even though it’s a GSM phone!

Dual-band phones

work with the two GSM standards in one geographic area – for instance GSM-900 and GSM-1800 in the United States. Tri-band phones work with the major

two standards, and the most widely supported standard elsewhere – for example, a Europian tri-band might support GSM-850 and GSM-1900 in Europe, and

GSM-850 in the US. A Quad-band phone should support all 4 major standards.

Data Services

Most phones these days provide

access to data services. Every carrier calls the plans something different. If you need data services, you should go for the unlimited data plan and

it should cost somewhere around $20 per month in the US. If you don’t know if you need data services, then you don’t need them – just leave it at

that.

The phones support many different types of data connectivity. Some let you browse the internet from your phone, but normally only a small

sub-set of pages that the carrier provides, which normally include sports, news, weather, maybe some shopping. Some of these interfaces are called WAP

pages, which is just a text-based web browser – actually it’s older technology that was supposed to keep us occupied until the networks could support

full speed data services – we’re kind of still waiting for that, although we’re getting closer all the time.

Some phones have a modem built in

- this means you can use your mobile phone as a modem with your laptop computer, normally either through a cable, an infrared port, or a bluetooth

connection. Don’t expect broadband speeds, they are more like slower dial-up speeds – and if you roam outside of the GSM network, which is still

largely centered around major cities and highways, you’ll drop your data services and loose connection altogether.

Still some other phones just

allow you to sync your data with your PC – so your contacts and appointments and notes and tasks all stay up to date. While they are called data

services, they are free, and just require the time and patience to install applications from the phone manufacturer. Once you get through that, you

can download MP3s to your phone, grab phone cam pics and email them from your PC for free (no MMC message charge for me, thanks), you can even install

java applications and games to many devices – they are available online and are normally completely free.

There are different network services

too, but by far the most popular is GPRS, the General Packet Radio Service. It works over GSM, and you can talk and use data services at the same

time. The next step is EDGE, which allows for about 3 times the data transfer rate over the GSM network. EVDO is another data service technology

which will allow for broadband-like speeds. Check with your preferred carrier to see which data services they offer, and also remember to check for

coverage of those data services, they may not be the same as your voice coverage.