Pressing any number in response to a robocall just confirms that they have a live number. Just hang up, report it to donotcall.gov and if possible block the number. These scammers change phone numbers more often than their stinky socks.
Here is some good info from the FTC
http://www.consumer.ftc.gov/articles/0259-robocalls#What_Should
Buy a Digitone, and quit thinking that the DNC list actually works. Better yet, next time Sally calls, just give them your CC number because it you are dumb enough to believe the DNC list actually works, you deserve what you get.
They call me 2 & 3 times a day. Thing is it isn't the same number but they say the same thing. I press the one to have them stop calling me and they hang up when you say "I don't have a credit card." But they still keep on calling. The FCC & FTC won't do anything about it because they are not calling you to sell something. I have already tried that with this number and another number. They say put your numbers on the do not call list but you will still get calls anyway.