888-276-0403

Called and asked for people not at this number. Was told this was a do-not-call number, and argued about it.

Sounds like this wasn't a sales call and if so then you perfectly well know that being on the list would not apply.

So your caller told you their number is a DNC number?  Maybe someone called them, spoofing your number with whatever name(s) they were asking about.

Apparently they didn't believe me when told that I was on a DNC number, because they've started calling again.