Legitimate telemarketing calls can substantially be avoided by consumers by signing up for the free federal Do-Not-Call list which makes it illegal for telemarketers to contact you on your landline phone or cell phone unless they are representing a charity, debt collector, survey or a politician.  Here is a link to where you can sign up for the Do-Not-Call list if you haven’t yet done so and want to avoid these calls.  https://www.donotcall.gov/
It is important to remember that criminal telemarketers, including many who utilize automated robocalls pay no attention to the Do-Not-Call list.
Now, new technology has been developed that enables telemarketers to contact you by calling directly to your voicemail without your phone ever ringing.  Telemarketers argue that this technology is not subject to the Do-Not-Call list because your phone never rings, which seems like poor reasoning to me, however, the Federal Communications Commission (FCC) is considering an application by a company that wants to use this technology for telemarketing to allow these calls to be made to consumers.
TIPS
Unfortunately, at the present time there is no way for you to block undesired voicemails as we have with other types of robocalls. The FCC is presently soliciting comments from the public as to whether or not to allow this new technology to be used for telemarketing to consumers.  If you wish to comment  to the FCC on this you can do so by clicking on this link.  You will need to insert the name and number of the proceeding as 02-278 Ringless Voicemail.
https://www.fcc.gov/ecfs/filings/express