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In Germany, people are allowed to move telephone numbers from one telephone network to another. But this is not a problem because it is easy to test which network a telephone number belongs to.

From T-Mobile network (and from T-Mobile discounters: Congstar, etc) dial: 4387
From Vodafone network (and from Vodafone discounters: smobile, etc) dial: 12313
From eplus network (and from eplus discounters: simyo, etc) dial: 10667

From O2 network (and from O2 discounters: Fonic, etc) send an SMS message to special service code 4636 with the text: "NETZ telephone number". Do not include the quotation marks. The SMS is free of charge.
Example: NETZ 01761234567

All of the above methods are free of charge, when they are internal network calls. These numbers can save a lot of money if you use a German flatrate tariff together with a second SIM.