092-585-3001. There were nine phone numbers in total to be found in the game. However, eight of them had an alternative number (depending on a specific scenario flag in your progress) providing the same audio message, so there were in fact seventeen active phone numbers in total to be found during the contest.
The alternate numbers route is triggered early on in the game, if you peep when the girls change their clothes in the classroom, but it seems the alternate “pervert” numbers did not change the contents of the messages when dialed. It’s possible the alternative numbers were meant as a way to stealthily separate prize winners in case too many calls were received on the same day (there were 16.000 prizes to be won in total), or that Nintendo and Square had initially planned to make use of this secret trick to admonish misbehaving players but had given up by the time the game released.
Some brave soul asked themselves and tested if the numbers still connected, back in 2015. It’s not stated if they followed the proper route’s numbers or the pervert route’s numbers, but based on the article, it seems to have been the proper route.
Unsurprisingly none of the numbers operate as intended back then, and seven of the numbers do no connect to anything, most likely due to the switch from a 9 digits to a 10 digits phone number system in Japan in 1991. However, two of the nine numbers still connect. One of them now* gets you through to a Toyota car dealer in Imajuku, while the other gets you to the reception desk of an office from some large industrial group in Sapporo (*as of 2015).