DO NOT CALL THEM BACK! This is how the "one-ring scam" works (from ScammersUncovered dot com): First you’ll receive a phone call from an unknown number. True to their name, they’ll ring once and hang up. If you call the number back, you’ll be put through to an international number who will charge you premium rates, and drain all of the dollars from your account. You won’t find out exactly how much has been charged, until your phone bill knocks the wind out of your sails.
The easiest way to prevent scam callers is by being aware of them. Never answer calls from  unidentified numbers. If you want to know who called you, use a Reverse Phone Lookup to safely identify unknown callers. Only when you know that the caller is someone you can trust, answer or give them a call back.
Most of these scumbags are offshore, so good luck trying to get any justice after the fact. The US Do Not Call Registry means nothing to them, unfortunately. The best thing you can do is to be wary of unknown phone callers. Always, ALWAYS use a reverse lookup when the number ends up in your phone log. If it’s an important call, the caller will leave a message — and even then, be wary.  I added the phone number to my phone book, and then blocked the phone number.  Never press any numbers, it also lets the caller know what time that you are home.

I have an unlisted number and am the Do Not Call Registry but still get 5 - 6 calls a day from these robo calls!

Received call on cell. Called back from a LAN line to get a prerecorded robo message to be added to their do not call list. No name or company provided. BLOCK.

Get a call from this number every few days. I usually press 1 to lower my card interest and find out that I am caller 8+ in line so I hang up.  Today I pressed 1 to lower my credit card interest and spoke with a real person! After my "Why the @#$% do you call me every couple of days, I do not want these phone calls, I do not have credit cards, I am not interested in credit cards" rant, I found the line dead. They hung up on me. Not wasting my time calling them back. So much for a donot call list.

I was so glad that this was the "final" chance to lower my interest rates. I might be safe until "Carmen" or "Rachel" goes to another company with a different phone call. I hate to listen while the phone keep ringing from these scams. I usually press one and then try to keep them on the line as long as I can. I figure if they are waiting for me, they aren't scamming someone else. I think that they have smartened up on this tactic. They don't stay on very long anymore. I don't think that I wasted a minute of their time this time.