erich wrote: Now I don't know exactly how the substitution is usually made (if there is a rule or people just use some of the native numbers randomly when they think the listener won't get the difference). I just remember having heard 하나 in phone numbers. Maybe some native can give more insight?
There is a rule of counting which is using by artillery.
1 하나
2 둘
3 삼
4 넷
5 오
6 여섯
7 칠
8 팔
9 아홉
0 공
They still use this counting system in order to avoid misunderstanding of the Fire Direction Center's command which may cause a serious mistake.
However, I don't think everyone is following this rule in ordinary lives.
Maybe they say randomly, I guess.