I hope you understand this is purely SQL*Plus isue - what you are trying to achive will work only in SQL*Plus. For any general solution, you would need to use the proper format mask with TO_CHAR function.
Anyway, if you insist on SQL*Plus with COLUMN formating command, then use this:
col purn for 0999999999MI
Your blank is reserved for a minus sign. With the above setting you are instructing SQL*Plus to display sign at the end of the string. Plus sign is not displayed by default, but if you have negative numbers the minus sign will appear at the right side.
Jurij Modic ASCII a stupid question, get a stupid ANSI
24 hours in a day .... 24 beer in a case .... coincidence?