The ISDN network is sheduled to be turned off by 2018 in Germany
(or at least somewhere in that region)
What the company I work at does, is to provide VoIP to people. We have 3 kinds of offers, the simplest one requires you to have your own network connection and equipment, the next one will get you an ADSL or SDSL line (without ISDN or POTS) depending on how many channels you want and a router, and the most advanced one will get you a CPE with up to 4 S0 (actually T0) Ports to connect your ISDN PBX to.
If you just want a telephone number in Germany you can use competitors like Sipgate which offer a free telephone number as well as pre-paid telephony. So if you have only a few calls, you can save a lot. However rules in Germany are different than in the UK, so the phone company might actually have to pay for a phone number. In Germany they are free, except for a small management fee.