For those of us who travel to Europe, the issues of cell phones are problematic. This is because chances are very good that the cell phone you have in the US will not work in Europe.

Spread Spectrum and CDMA
There are two major mobile access technologies. One is called Code Division Multiple Access, or CDMA. It is also known by the moniker 'spread spectrum'. The spread spectrum technology was actually patented in 1942 by a Hollywood bombshell named Hedy Lamarr but was very ahead of its time. CDMA uses unique 'codes' and transmit over a wide frequency range. This is like multiple groups talking to one another in a bar, all using separate languages. You may hear other's people conversation, but your brain tunes them out because it's in another language. Unfortunately, though CDMA was arguably a better technology, it wasn't widely adopted until cheaper processors became ubiquitous. Since the US developed its digital phone technologies later than Europe, its most service providers decided to use CDMA. Companies that use CDMA include Spring and Verizon Wireless.

Time Division and GSM
The other major access technology goes by the moniker of Time Division Multiple Access, or TDMA. This is akin to letting one person in a room talk at one time in a round robin fashion. This was also the access technology chosen by the Europeans when they designed their digital phone standard, GSM (Global System for Mobiles). In all of Europe and other countries such as Australia, GSM is the standard for cell phones. There are some companies in the US that uses GSM on its own networks, including AT&T, Cingular, and T-Mobile.

Just because you have a GSM phone...
Now if your cell provider is T-Mobile, Cingular, or AT&T, don't think you can simply take your phone to Europe! The Europeans operate GSM on 900 and 1800 MHz (the latter is frequency is nicknamed DCS, or Digital Communications System), respectively. We Americans decided to use GSM on the 1900 MHz frequency, also known as PCS (Personal Communication Services). More recently, some American phone companies started support GSM on 850 MHz in rural areas. This is because lower frequencies (and longer wavelengths) are more efficient in less dense areas. So, if you want a phone that truly operate anywhere in the world (except Japan), you need an unlocked (that is, you can use it other mobile networks) quad-band GSM phone, like the $650 Motorola Moto Razr V3.

Maybe you should just get a different phone...
Frankly, GSM coverage in the United Stated is spotty at best. Even the best GSM phone will not get as good coverage as the best CDMA phone under, say, Verizon Wireless. Given the cost of a good quad band unlocked phone, you are really better off simply buying a small phone for travel, and get a CDMA phone in the US for coverage.

What to look for in a travel phone...
Assuming that you're buying a phone for Europe, it should at least support GSM on the 900 and 1800 MHz frequencies. It should be small, and the charger should support multiple voltages and power cycles. The phone needs to be unlocked, so you can go anywhere, by a SIM card with, say, 10 Euro on it, plug the card into your phone, and talk until you deplete the account. You can then replenish the card with additional funds.

The best phone I've found is, (surprise, surprise), the Sony Ericcson T100.

Major Features
The T100 is a very small and light phone (according to the packaging, it weighs 75 grams and is 99 x 43.5 x 17.7 mm in size), being the same size as four compact flash cards stacked in a 2x2 configuration.

It supports GSM 900/1800, WAP (Wireless Access Protocol, every GSM phone supports this), and has good SMS (Small Message Service--basically, text messaging) and EMS (Extended Message Service--basically, extra long text messages) support. Good SMS support is very important, as in Europe, people like to send messages to each other without actually calling.

The battery has a 650 mAh capacity, enough to last 3 days in stand by mode, or about 2.5 hours in talk mode. The included charger supports both 110 and 220 Volts, as well as 50 and 60 Hz power and thus can be used anywhere in the world.

Voice Quality
I have no complaints about the voice quality, which sounds much like any other phone I've used. The only complaint is that the microphone located at the front instead of the base of the phone. You can easily cover up the microphone if you're not careful.

Languages and SMS
I've actually heard others complain about the difficulty in entering text due to the small size of the keys. Since I have small fingers, this doesn't bother me. The phone itself supports the T9 input method with English, Russian, and Ukrainian dictionaries. That is, if you hit 2, 6, 3, it will guess you're trying to type 'and'. It is pretty easy to use, and a must if you use SMS often. The phone itself supports menus in English, Russian, Byelorussian, Estonian, Latvian, Lithuanian, Romanian, and Ukrainian.

Interface
The phone has a standard Sony Ericsson interface and is very intuitive. It took me no time at all to become comfortable with the phone.

WAP
Basic WAP support (WAP v1.2.1) is provided and supports data connections of up to 9.6 Kbps. I have not used this feature and can't comment on it.

Other Features
This phone comes with a calculator, alarm, and a very basic calendar. With a cable, you can download edited greyscale pictures to the phone, and even use it for caller identification. The melody editor for ring tones absolutely bites, and supports only a single tone at one time, though I care little for ring tones.

Pros and Cons
This phone is cheap (I paid 1690 rubles for this phone, or about $60), and is very portable. The reception is decent, and I can often use the phone even in the metro (subway). The down side to the phone (or a plus if you like simplicity) is the lack of extra frills. Also, the phone book does not support multiple phone numbers per entry. The memory capacity for the phone is also very limited, allowing only 100 phone numbers and something like 20 SMS messages before you have to delete some of them due to the lack of space. But hey, for 1690 rubles, it's a good trade off!