Credit Merchant robocaller scam

Didn't recognize the number, let it go to Voice Mail. After 2 rings, call dropped. Telemarketer's trick to urge a callback, and when you do, your number is "valid" to be sold to other telemarketers. Never call a dropped call!

Another Annoyance call to my registered do not call cell number.  It's interesting that they only call that and not my land line.  How do we get them to stop?