Why did this guy say he’d ‘let me know’ about calling me?

I’ve been talking to this guy I liked when we met during university and we are just friends now as he doesn’t feel the same…well he didn’t years ago. We are talking again after a few years and he’s always nice and responds quickly to messages etc but doesn’t initiate.
I said to him to give me a call when he’s ready and he says ‘Not now but ill let you know sometime in the future’ with a smiley…I didn’t respond- I get it that he probably doesn’t intend to and he’s busy etc but why even say he’d let me know in the first place?

Why did this guy say he’d ‘let me know’ about calling me?
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