The former captain was caught by surprise with a call from the Blues’s new £34million signing and had to contact manager Thomas Tuchel to verify that it was really him.
Speaking at his unveiling press conference at Chelsea’s Cobham Training Centre on Wednesday, Koulibaly revealed the details of the exchange and how fellow legend Gianfranco Zola even got involved.
“The team manager told me some numbers but he didn’t tell me 26,” Koulibaly told reporters. “I asked him for 26 and he told me that John left in 2017 and no one took it all this time.
“I asked whether it was retired or no one wanted to take it. I asked Gianfranco Zola because we know the same person who works for a business in Italy. I spoke to him because he was the one who told me the story of Chelsea.
“I always want to know the full story before I go somewhere and I asked him a lot of questions about the club, about the supporters and the number of John. I wanted to ask about the number for Chelsea.
“He gave me his number directly. I called him and at first, he didn’t believe it was me - he thought it was a joke. He then called the team manager to ask if it was really me.
“After, I asked him again and I told him fully that I wanted to take his number. I know that for him it is a really important number. It is also an important number for me as I took it in Napoli and I wanted it again here in Chelsea. When he told me yes, I was really happy.
“I know what he did for the club, the supporters and the city here. I know that it is very important to ask him first before wearing it. I preferred to ask him and get his answer and he was happy. I wanted to tell everyone that I asked him before because I don’t want to be disrespectful to a legend in the club.
“I told him that I have to ask him before wearing it and he said yes and I am very happy. But he put the phone down at first because he didn’t believe it was me.”