[6] 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.
[4] "Unknown Caller" made its live debut on the opening night of the U2 360° Tour with Larry Mullen Jr and The Edge both providing backing vocals.
[14][15] Q described the song's placement in the opening half of the album as "the U2 of wide-open spaces, of sweeping mountain valleys, and of Edge's signature chiming guitar lines.
"[16] Mojo felt that bassist Adam Clayton's playing was largely responsible for the song's "evocation of both frigidity and tenderness", noting the transition from the French horn to The Edge's guitar solo was "as exalted as any U2 music gets.
"[8] Blender felt that Bono "reache[d] Bowie-in-Berlin levels of arty alienation," and labelled the guitar playing in the song the best from the album, while the New York Times believed it would be a "likely arena singalong" live.
[17][18] Uncut likened the song to the band's 1984 single "Pride (In the Name of Love)", describing it as "the most dramatic bait-and-switch on a record riddled with them – a gentle Edge guitar figure and birdsong an unlikely foundation for the gradual erection of a terrifically unabashed stadium epic.
[6] Dutch newspaper NRC likened the end of "Unknown Caller" to a JS Bach composition, stating: "The chorus unfolds as a militaristic polyphonic speech: 'Shout it out/ Rise up/ Escape yourself,/ and gravity', and then undergoes a baroque transformation - guitars become horns, harpsichords become organs - for which J.S.
"[20] Allmusic was not a fan of the song, stating "when U2, Eno, and Lanois push too hard – the ill-begotten techno-speak overload of "Unknown Caller"... – the ideas collapse like a pyramid of cards... turning it into a murky muddle.
"[21] NME believed that the lyrics were No Line on the Horizon's greatest weakpoint, citing "Unknown Caller"'s chorus of "Force quit and move to trash" and "Restart and reboot yourself" as being inspired by Bono's computer.