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From wikipedia:

The band is notoriously reclusive, as they reject to accept mail and allow no visitors at the Kling Klang Studio. Another notable example of this eccentric behaviour was reported to Johnny Marr of The Smiths by Karl Bartos, who explained that anyone trying to contact the band for collaboration would be told the studio telephone did not have a ringer, since during recording, the band did not like to hear any kind of noise pollution. Instead, callers were instructed to phone the studio precisely at a certain time, whereupon the phone would be answered by Ralf Hütter, despite never hearing the phone ring.