Hi @Anonymous there seems to be s solution in this thread
Hi @Anonymous It was the same issue on this thread and problem solved
Also scroll down on this article. Plenty of tips given there https://www.imobie.com/support/how-to-fix-voicemail-after-updating-ios-10.htm
Veritas Numquam Perit
Thanks for raising this to us last night and this morning. We're currently investigating the cause of this, but in the meantime what we have seen to work is sending a message saying "STATE" to 9011. This will reset the mailbox, but won't delete any of your messages.
If that doesn't work for you, please let us know.