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"Unknown Caller" is a song by Irish rock band U2 and the fourth track on their 2009 album No Line on the Horizon. It was written from the perspective of a drug addict, who begins to receive bizarre text messages on his cellphone. The song was developed very early during the No Line on the Horizon sessions, and was recorded in a single take.
The No Line on the Horizon sessions began with two weeks of recording in Fez, Morocco.