As far as I know, no one I know has those numbers. They leave no messages.
A search on the internet for phone spammers reveals nothing. It does confirm that Costa Rica went to eight digit numbers last year, so these appear to be valid numbers.
I don't really want to call them back.
Edit: I love the intartubes. I just got more phone spam (this time from within the States) and while the phone was ringing I looked up the number to verify that it had been reported as a spamming number.