I also got a call on my mobile from this number. They must be the ones calling my office line (no Caller ID there). I may owe Capital One, but I'm current on my bills. No need for collections on me. I think I went over my limit once in the past 6 months which I took care of via their web site.
If this really is Capital One (or related company), I *WILL* close my account when the card is paid off.
I bet it IS NCO because they've been calling me from a variety of phone numbers (at least 9 now, from different parts of the country). They very rarely leave a message, and the only time they do is when they're calling from their main 800 or 866 number. Trying to cover their tracks?