Ignore it.
Even replying with "who's asking" is kind of a confirmation.
You don't know if they're trying to phish for information, run a scam, or whatever else, so it's usually best not to respond.
If I have time, I might do a quick check on the number to see if it's coming from a legitimate cell carrier, and if it appears in any past correspondence.
They're contacting you--it's on them to identify who they are and the reason why they are contacting you.