Hi Wolfi
According to my information the LDP-DPB is the new Door Phone Box from LG-Nortel.
It can be connected to any ipLDK DTIB (keyset) board, and has its own station number.
If you press the Call button and have programmed a hotline for this station number, then the phone "in the other end of the hotline" will ring. A conversation can now be held between the DPB and the e.g. keyset indside the building.
A door phone is normally placed outside or in an un-manned reception.
If it also can work as a music source, I don't know...
///doktor