I assume your phone does not ring, if so blocking the number even if you could would be ineffective. Since the phone never rings NOMOROBO is similarly not effective. Spammers have a new "technique" of dropping voicemails directly into your voicemail mailbox without ringing your phone. Registering a complaint with the FCC is the only thing you can do right now and if they get enough complaints will respond. Ultimately technology will have to be updated to block this type of spam.