From New York Post (caution:tabloid):
The secret conclave that elected Pope Leo head of the Catholic Church last May was interrupted when one of the 133 cardinals involved was found carrying a cellphone, a massive security breach, a book released on Sunday revealed.
As the clerics were preparing to take their first vote inside the Vatican’s Sistine Chapel, which was fitted with jamming equipment to prevent outside communications, security officials picked up the signal of an active mobile connection. The cardinals stared at each other incredulously, then one of the older clerics discovered he had a phone in his pocket and handed it over, according to “The Election of Pope Leo XIV,” a new book by two long-time Vatican correspondents.