Koulibaly, 31, signed for the Blues in a £33million deal earlier this summer.
The Senegal centre-back established himself as one of the best defenders in world football during an eight-year spell at Napoli.
He wore No.26 throughout his time in Italy, but that number has been left vacant at Stamford Bridge since Terry's departure in 2017.
The former Blues captain won five Premier League titles over a 22-year career at the club, cementing himself as arguably the greatest player in Chelsea history.
Out of respect for his predecessor, Koulibaly picked up the phone to ask Terry for his shirt number.
And the Chelsea legend was happy to oblige.
In a clip shared by Koulibaly, Terry could be heard saying: "The 26 was very special to me, I really appreciate you calling.
"It's no problem for me for you to take it, absolute pleasure.
"I wish you well because I know how important the number was for me and I've seen you wear 26 as well.
"My pleasure mate, I give you the 26.
"No pressure. You go and keeping winning trophies for us."
Koulibaly has been wearing the 26 shirt for Chelsea in pre-season and recently shone during a 3-1 victory over Udinese.
He responded: "It will be a heavy shirt but I will do my best."