Reading zero as O (fifteenth letter in English) [duplicate]
Solution 1:
The number zero is usually pronounced that way when "spelling" out a number, not only in telephone numbers.
As Sandeep Dhamija's link shows, it's also used in many other cases, like in James Bond's call sign double-oh-seven: 007.
Mind you, when it is important that whatever you are spelling out is understood correctly, even over a not-so-good communication line, the digit zero is pronounced as "zero". Actually, in that case, 4 can become "fower" and 9 can become "niner" to facilitate understanding by the receiving party.