Phone number or other contact info. Supposing you’re on good terms, they’re not someone you’re interested in romantically or the like (so none of the anxiety from that at least), what’s kept you from asking?
Supposing that something has on occasion, anyway.
Non-romantic?
I ask for something more low-key like a method to contact them in the future. Typically LinkedIn or originally Twitter. That would lead to email info.
I don’t ever bother with phone numbers. Because personally, I don’t want to be texted/called on the phone myself.
Fwiw I sometimes do the same, but as mentioned in another comment, I haven’t settled on a good way to phrase asking that doesn’t sound clunky af, or end up going down a process of elimination of which sites/services you’re both on & down for using.