When a Business Model Goes Rogue
Getting Your US Phone Unlocked: Most mobile phones sold in the US are electronically locked to work exclusively with the carrier that sells them. However, any GSM phone — which all use SIM cards — can be unlocked for use with other providers (remember, some phones from Verizon or Sprint don't have SIM card slots). Just call your provider, ask if your phone will work in Europe, and see if they will send you an unlock code. If they agree, you'll receive a long code that you can punch into your phone. Some providers are willing to give you a code after you've been under contract for 90 days, while others wait until your phone's contract has expired. (Sometimes they won't do it at all, though companies seem to be gradually loosening up on this.) You can also go through back channels to get an unlock code (either on the Internet or at a back-alley mobile phone shop), but this is less reliable and in some situations may even be illegal.
Buying and Using SIM Cards: Once you have an unlocked phone, you'll need to buy a SIM card to make it work anywhere in Europe. A SIM card is a small, fingernail-size chip that stores your phone number and other information. With an unlocked phone, you can buy a SIM card at your destination and have your very own European phone number at local calling rates. I've bought SIM cards for my unlocked phone in two dozen different countries, and it's become a convenience I can't live without.
While some online companies in the US sell European SIM cards, these tend to be outrageously marked up (to prey on nervous travelers who don't realize how easy it is to buy SIM cards in Europe). For the best deal, just buy one when you arrive in Europe. Each country has various service providers, all of whom sell their own SIM cards. Since these companies are very competitive, they're pretty much the same — just look for the best rates. SIM cards, which generally cost around $5-15, come with a European phone number and starter credit. These days, mobile-phone companies are working hard to attract customers, and you'll often get the SIM card free when you buy calling credit. I've even bought a few SIM cards that came with more credit than the cost of the card (for example, a €5 card that includes €7 of credit).