If this works for somebody, i used it in Mexico with Telcel as carrier and a Galaxy S8. I had canceled the option of voice messages with the carrier, but i ALWAYS had the icon showing as if i had a voice message, dealed with that for months just closing it from settings/apps/Phone, then after months with that bugging me. I decided to contact the carrier, they said they couldn't do anything cause i cancelled the service, then i asked.. ok then please activate it again which they did, i registered my name, welcome message as if i was gonna use it... and then the icon dissapeared!!.. i immediatly contacted them to cancel the service again and voilá! the icon finally has gone.
This was finally a permanent solution, hope this can he helpful,