Here's a first for me - and anyone reading this should be prepared to get a ton of these calls from now on. There will be NO phone ring and NOTHING will show as an incoming cell phone notification that you've gotten or are getting a new call... but you DO find a new voicemail.
And what's in the voicemail? Someone's telling you about a new marketing technique that boasts about the no-ring voicemail-only call that they just slipped by you, and how you should PAY THEM to tell you how to "cash in" by using it for your own marketing campaign. They are obviously targeting cell phones, since the caller states that you should "redial the number showing in your voicemail notification". Seems like someone has rewritten the calling cue routine for robo-dialing software that will limit the outgoing calls to only dial known cell phone numbers.
The good news... You will at least see the dialing number after the fact as a voicemail notifier, and it will also show in your 'recents' call log and voicemail list.
The bad news... As these 'marketing schemes' begin to be abused by all the usual robo-dialing dick-heads, they'll flood you with enough calls to fill up your voicemail storage and make it impossible to rely on getting legit voicemails.
This appears similar to a pyramid scheme some canned "coach" clown tried to push on me last month, also the subject of a forum thread. What I got prompted for a keypress and could be far more agnostic as to the types of phones targeted.