Converting an ANNEX B router into ANNEX A by adapter

[Zio Peppe]

A friend of mine got a Billion BiPac 7005G modem/router for ADSL over
ISDN (RJ45)
Back from Germany he wants to use this router for ADSL over an analog
POTS line (RJ11)

HOw to convert signals between two standards?

Thanks,
Zpp