Europe's cell phone system operates using the Global System for Mobile (GSM.) It's the most popular mobile phone system in the world. In the USA two major carriers do not use GSM, they are Sprint and Verizon who both use Code Division Multiple Access (CDMA.) The two systems, GSM and CDMA, are not technically compatible with each other, although you can make phone calls between them. That means that if you have a regular Sprint or Verizon cell phone, it will not work in Europe. Even if you have a GSM-compatible phone from AT&T or T-Mobile, for example, you may want to use a local phone or SIM card for cost savings.
Both Sprint and Verizon offer world phones which have GSM-compatible radios. The Samsung Ace from Sprint has both a CDMA radio and a GSM radio. The device has an unlocked SIM card slot, which allows you to purchase a pay-as-you go SIM card when abroad, insert it in the phone and take advantage of low calling rates with a local phone number. This enables you to use one device when traveling. A disadvantage to this phone is that, although it offers EV-DO high-speed Internet in the USA's CDMA market, it's GSM data speed doesn't quite cut it and the phone can't take advantage of the highest speeds available in Europe called UMTS.
Local Pay-As-You-Go Phone
The simplest and most economical solution to making and receiving cell phone calls in Europe is to buy a cheap pay-as-you go phone. You can buy one on any main street, in pharmacies, newspaper stores and dedicated phone shops. They can cost as little as GBP 4.95 with a purchase of GBP 10 of airtime for the Nokia 1112 on UK network Virgin Mobile. Calls cost GBP 0.20. Roaming with a US phone in the UK could cost you $1.29 US a minute (Sprint.)
Local SIM Card
If you have a GSM phone from a US provider like AT&T or T-Mobile, you can get the phone unlocked when you're abroad and buy a local SIM card. That way you get the advantage of your own phone, complete with your address book. You will find stores that offer unlocking near any railway station or city area where tourists congregate. The shops will have signs in the windows offering the service. Once the phone is unlocked, pick up a SIM card from any phone shop. They'll cost about GBP 5.00 but will come with some airtime. The card will have a unique phone number. You just remove your US card—keep it somewhere safe for when you return to the US—and insert the new SIM card in the slot. Your US phone will become localized, complete with local rates and a local number.
Calling Back to the US
European phone shops have SIM cards specifically geared towards international calls and at special rates. Just pop them in the phone the same way. They tend to be free when you buy GBP 10 of airtime. Rates to the US can be as low as GBP 0.04 a minute.
Be very wary of using the Internet when roaming internationally. Data can be very expensive. Always use a local SIM card for data and check the rates. T-Mobile offers unlimited data using their SIM card for GBP 0.50 a day in England.