Stefano Cabizza was shocked when he picked up his phone at home in the north-west Italian city of Padua, only to find the Pope on the end of the line.
The information engineering student had written a letter to the pope and gave it to a cardinal when he attended mass with his family in Rome.
“I wanted to speak about my fantastic experience to bring this great and humble gesture to light, and the closeness between us faithful followers and Pope Francis,” Cabizza said, quoted in Il Gazzettino.
The pair spoke on the phone for eight minutes, during which the Pope told Cabizza to address him with the informal "tu".
“Do you think that the apostles used ‘Lei’ with Jesus? Or used ‘your excellency'?
“They were friends like us now, and friends are used to using ‘tu’,” the Pope told the teenager, according to the paper.
The ‘Lei’ form of ‘you’ in Italian is used as a mark of respect and it would usually be unthinkable for a teenager to address the leader of the Catholic Church with the casual 'Tu'.
Since becoming pontiff in March, Pope Francis has been praised for his humble and informal style which has helped him reconnect with Catholic youth.
Last month he attracted millions of young Catholics to the beaches of Rio de Janeiro, as part of an international youth festival.
In July the Pope also personally responded to a three-year-old, who had written a letter to the pontiff and delivered it with the help of her grandmother.