0721 can be pronounced in Japan as "o-na-ni-i" (o = 0, na = 7, ni = 2 = i = 1) “Onanii” and used to describe masturbation.
Interestingly the numerals, “0721” is “(I) Love You” Number ‘7’ represents the total number of letters in the term. Number 2 denotes the number of words and the numeral ‘1’ refers to “one meaning”. You can have a doubt as to why the numeral “0” is prefixed. That’s just like that. “Love You” may not be complete and hence, the letter “I” has to be prefixed.