Line Hold Music
While the operator is busy handling the current traffic, additional incoming calls can be deferred to special "line hold music" channels.
This feature was introduced at the Gulaschprogrammiernacht 2016. It was implemented by using an old MP3 player and the additional hardware described here. By separating the left and right audio channels in hardware and combining it with a specifically crafted audio file, a single player can provide two independent music channels.
For the 33c3 the same functionality was integrated in the "special services box". There, the MP3 player is replaced by a RaspberryPi (1st gen, model B) and a USB sound card, using the same audio files than the previous GPN set up.