left empty voicemail

I keep getting these robot calls from variations of this number.  How do they get away with it when it is illegal to use automated messages to cell phones without prior permission? It's always that I won some kind of vacation worth thousands of dollars

Calls occasionally, never leaves message, the DNA evidence of a scammer telemarketer. Now blocked.

Use variations of different numbers calling my mobile phone. They call with no one on other end or doesn't leave voicemail. Same phone/carrier company though:  360NETWORKS (USA) INC. - CA. Numbers they called on my phone: 213-973-6402, 213-973-6417, 213-973-6490. Last 4 digits change all the time. I'm on the DO NOT CALL list for two years already.

Left no message.