If you're going from US/Canada to Europe, ideally your phone should run on 900Mhz and 1800Mhz and be unlocked so you can insert another carriers SIM card.
Two frequency bands are used by GSM services in the US. Two different frequency bands are used by GSM services elsewhere in the world.
There are two common tri-band phones:
900/1800/1900 - Excellent internationally and very good in the US
850/1800/1900 - Excellent in the US but not very good internationally
The best of both worlds is to have a quad-band.
US: 850 MHz, 1900 MHz
Europe and most of the rest of the world:
900 MHz, 1800 MHz
I'm with T-mobile and I can use my SIM card abroad and keep my US number, but airtime from Germany is charged at 99 cents a minute.
To save on calls I would get another pre-paid SIM card there, but then I would have a different local number instead. T-mobile has several pre-paid options in Germany. Their brand name for pre-paid is "Xtra":
Xtra-Classic: .39 Euro per minute, .19 Euro per SMS
Xtra-SMSFriends: .39 Euro/min, .05 - .15 Euro per SMS, .05/min to a number you designate.
Xtra-Plus: .09-.79 Euro/min depending how long you talk - in/out of network, etc., .19 Euro SMS
The cards cost 19.95 and come with 10.00 Euro of talk time. After that you can just get "XtraCash" to recharge the card.