特にモービルでD700などを使っているときは普通のメッセージを打ち込んでいくのが難しい、そういう場合の短縮形です。
> 0 - NO
> 1 - YES
> 2 - Can't talk now - XYL is watching and volume is turned down (getting in 2 trouble?)
> 3 - BRB - be right back (3 == B)
> 4 - hi hi (H/h looks like 4)
> 5 - Meet you at the normal (frequency/coffee shop) (5 == F)
> 6 - What's happening? Que pasa? (6 == Q)
> 7 - 73's, good bye
> 8 - Honk - Jab (5+3 Hi Dave)
> 9 - Poke, nudge, hey there! (9 == P)