Those calls are a ping call, just to check if there is an answer once you answer the call, you will receive another call from different number. I just block the calls, everytime I get a call like this I just let it go to the voicemail and if they don't leave a message that means another number to my blocked numember's list.
No worth it to get upset, I have been blocking numbers for the last 8 months and I got used to get this calls, I can only suggest to be patience and block the numbers this is the only way since the DNC does not work.
Got a call from this number with no voicemail message. Called back, got a strange automated message stating "If you received this call, we should contact you within 24 hours."