It looks like the prefix may be coming from Canada or the Caribbean, which means the Do Not Call law doesn't cover the phone number - other countries don't have to abide by our laws in that regard. The best thing to do is not pick up, otherwise they may track what time you answer, and then keep calling around that time. Another possibility is that they are selling your number for future telemarketing purposes, and want to know if the number is valid, and when you will be available to answer the phone. Best not to answer ANY number that isn't familiar to you.