[Motel75]

Second what Inquisitor said. You can get a prepaid phone and SIM in Germany for as little as 5 euro, and for example the standard Vodafone 5/15 tariff is perfectly fine for calling within Germany (there are cheaper SIM-only offers, of course). However, if you want to place cheap international calls, Solomo is the way to go - you'll need a phone for the SIM, so either bring yours with you or buy an unlocked one (places like Saturn and Media Markt usually have a basic unlocked Nokia for about 25 euros).

And don't even think about renting a phone - the only companies that do this are a ripoff for foreigners who don't know any better. A colleague came to visit from the US and rented a phone for 40 euros a week - it was the same prepaid deal you can get in the shops (at the time there were prepaid packages everywhere for 20 euros or less). And she obviously had to give it back at the end to avoid being charged a lot of money to "replace" it.