To build one yourself is most likely not cost efficient. Be aware of possible legal conflicts. Your phone is more likely to be illegal than the homemade adapter. The most convenient way for German tourists to call home is the Deutschland direkt Line. The cost is supposedly around DM 2.10 per minute, plus a flat fee of DM 3.00.
Collect calls are possible.
It is also possible to use a special phone card and password. Ask your local 'telefonladen' for details. 130' numbers are Germany's version of the US '800' numbers. But if you call them from another country you will have to pay the usual fee for operator assisted long distance calls. It is also not possible to reach US '800' numbers from Germany.