I'm having the same problem on a Google Pixel. Since updating to Android 8.0 (Oreo), I'm getting cryptic text messages instead of a voicemail indicator. Searching the Verizon site, the only thing I can find is that Verizon "thinks" I have visual voicemail and I should go remove it. I don't have visual voicemail, and it's no where on my account to delete. Does Verizon just need to catch up with the new OS? This is annoying.
Go into the dialer application and in the settings turn on and off for the visual voicemail under the voicemail options.
If you want to only get the basic non-visual voicemail feature, I would also suggest that you call customer service at 1-800-922-0204 from a landline or *611 from your phone and request they add the Block Visual Voicemail feature to your line.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.