I saw a database once that stored phone numbers as ints. elsewhere in the software, they tried to take the mean of the wrong column. If the phone numbers had been stored as strings the error would have been obvious. As it was, the error survived in production for a disturbingly long time. The users persisted in thinking the output was meaningful even after the problem was discovered.
I'd honestly have rather it stored a NULL than that result though.
Don't ever do this.
Although, the other reason that won't work (which I thought you were going to say) is that it'll end up as -X1XX!
In St. Louis had 314 and 636 area codes, sonetimes they needed a 1 in front of them based on where you called from. 636 was west county like St. Charles 314 was St. Louis city and North, East, and West county. They used a different area code to make more numbers due to demand.