Jeremiah 33:3
3 Call unto me, and I will answer thee, and show thee great and mighty things, which thou knowest not.
"Interestingly for the album’s cover, the name on a departure gate “was changed to J33-3, in reference to the Bible verse Jeremiah 33:3, "Call unto me and I will answer thee great and mighty things which thou knowest not." Bono referred to it as "God's phone number"”, and was still employing the number with “Unknown Caller”." (Wikipedia)