The link below has many useful info - to check out operators, bands used & prepaid options. You will find T-Mo in U.K. and Germany but USA T-Mo PP sim card will not roam international at all, except for Mexico & Canada. For Germany & France, it's partnering with Vodafone - might be worthwhile to get their prepaid sim card.
Roaming calls & calls to the US using prepaid sim can cost easily 2.00 Euro + per minute compared to inexpensive local calls on their native network - as mentioned, unlocked quadband phone is a must - and data/sms charges are extra options, not a given. Euro networks are usually 900/1800 and WCDMA 2100 vs. North America's 850/1900 & WCDMA 1700 - research your existing phone's specs & plan accordingly. Buy a local prepaid sim kit upon landing oversea is often the best option - some of the global sim card offered on the web are overpriced & not worth the money.
Have a safe trip ....
NW.Fam.Share 1400 F&F, Unl. In-Call & N/W, Opera Mini/Skype w. WM6.5 & WiFi access, Goggle Mobile Maps/GPS. AT&T MEdiaNet & T-Zone on GSM. On the Job: UHF 5W Duplex HT DTMF-PL repeater (backup: 800Mhz Motorola & Nextel portables) - private 900Mhz alpha-numeric paging net for 24/7/365 Incident Command - Is your "Go" Kit ready? Mass Transit Tips: "If you see something, say something - Call 911 !"