How To Initiate A Conference Call

How long should I wait to call him after he finally gave me his number three weeks later?
Ok so he’s in his early 30’s, I’m 23.We met at this conference a few months back. He was feeling me and I was attracted to him. He initiated personal connection between us which eventually led to flirting. On the last day of the conference he told me to email him. I did with a funny yet direct email. I had no answer for 3 weeks. He finally emailed me back and sent his number. I am going to call him that is a fact but to not seem anxious or at his beckon call; how long should I wait to call him?
Mind you he is 30 something and could have genuinely been busy but three weeks after you are the one who told me to make sure to email you? How long should I make him wait?
Should I be very casual or inquistive when I talk to him for the first time.
Meaning should I ask what took so long or should I just move on and talk about casual topics??
amm you know the 2 day rule???? so ya 2 days lolzzzzzz