Indeed, why give someone an old number. If you are concerned about meeting them again and want plausible deniability, transpose a couple of digits and swap a 1 for a 7.

Or if you're musk, pay a personal assistant to field calls from people you don't want to talk to

That’s also the classic method of getting in touch with a recruiter or hiring manager. Call the company and state “Someone called me the other day from X company and left a voicemail, they did not state their name so I was just following up”.