Re: The ISDN network is sheduled to be turned off by 2018 in Germany
There isn't necessarily a monthly fee for a phone number in the UK, though most VoIP providers would doubtless like you to believe that's not the case.
Typically, they'll charge you a few quid a month for a geographic phone number, including on you've ported from another phone company (such as BT).
On of the reasons I went with the VoIP service that I have now (which is Gamma telecom, resold by my ISP) is that while there was an initial fee for the number port, there wasn't an ongoing charge per number. Since I was porting my ten number block from the ISDN line, at some providers charge £2 or more per number, per month, that was a fairly important consideration.