"Unknown Caller" is the fourth track from the 2009 album No Line on the Horizon. According to The Edge, "the idea is that the narrator is in an altered state, and his phone starts talking to him".
| Year | Covered by | Album |
|---|---|---|
| 2009 | Vitamin String Quartet | The Vitamin String Quartet Performs U2's No Line on the Horizon |
Read more about this topic: List Of Cover Versions Of U2 Songs