On a mildly hacked on asterisk server, I was able to set this up and drop voicemails in every number I tried (I owned or with permission) in about 40 minutes. I didn’t use any of the methods mentioned above. Just because you can do something, doesn’t mean you should. Yes, of course Asterisk can be used for nefarious purposes, however I, and I think you (whomever may be reading this) will find I and almost everyone here is going to be against giving advice on how to spam.