"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. Recording took place in a rented courtyard of a hotel Riad, which the band turned into a makeshift recording studio. "Unknown Caller" was recorded in a single take in this time, along with the songs "No Line on the Horizon", "Moment of Surrender", and "White as Snow". A few iterations of the track had previously been developed, but the "definitive version was only ever played once." Guitarist The Edge noted that they "were songs that pretty much came together in the space of a couple of hours, and therefore probably were played in the final incarnation maybe once or twice." Co-writer and producer Daniel Lanois noted that the song "pretty much had its personality intact from day one." The open-air Riad allowed the band to hear the overhead birdsong during their sessions; this was taped and included in the introduction of the song. Towards the end of the recording sessions, producer Steve Lillywhite made a few minor changes to the song to make it "brilliant", including changes to the song's drumming and bassline.

In an interview with The Guardian, lead singer Bono stated that he became tired of writing in the first-person, noting that "I'd just worn myself out as a subject matter"; as a result he created several characters, including a traffic cop, a drug addict, and a soldier serving in Afghanistan. The drug addict character appears in "Unknown Caller", as well as in "Moment of Surrender", when the character is having a crisis of faith and is suicidal. In an altered state, the character attempts to use his phone to buy drugs, when he begins receiving cryptic text messages with technology-inspired directions. The themes in the song include social alienation and personal identity, as well as optimism.Mojo noted that the drug addict was "not unusual in this record in being lost, spiritually broken." The lyrics "3:33, when the numbers fell off the clock face" are a reference to Jeremiah 33:3, the same Bible verse referenced on the cover of U2's 2000 album, All That You Can't Leave Behind.

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