How did my ex boyfriend know?

My ex boyfriend (of 2 years) came back to my state month ago so he contacted me
He told me he got into a relationship after me
But he's telling me has trust issues since she left him
But we were close so he still trust me abit and only talk to few people now
But he gave me his new number

Anyway so me and my friend were bored yesterday so I prankcalled him
- just called on private number and waited for him to say "hello" 3 times and then hung up

Few days later he messaged saying
"Hey did u call me on private number? Lol I know you 😂 you use to do this type of stuff"
But I denied it

How did he know it was me?
Does it mean he does not get private calls?
Or does not give out his new number to much people?
Yea it means he doesn't give out his number to just anyone
Just know you well
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How did my ex boyfriend know?
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