I have this problem also. After more than an hour with a phone tech, including a screen sharing session - he was stumped and is looking into it. I will post the solution if it happens. Meanwhile, please send feedback to let Apple know how dumb this is. My feedback to him and apple.com/feedback was -
1) Instead of telling me MY cell # is "assigned to another Apple ID", tell me WHAT Apple ID. It should be "123-456-7890 is already assigned to Apple ID ************@mac.com". Then, at least I can either report the illegal use of my cell number by a stranger OR contact the family member or employee to unlink it (could have happened before VoicePass existed if someone picked something up for me from an Apple Store).
2) Even easier solution - Verify ownership of any cell number added to VoicePass with a text, containing a code - same basic security procedure used by alost every other apps and site.
I'm not annoyed by the problem. Things change. I'm annoyed that we have to jump through hoops to fix something that we should so easily be able to fix.
<Email Edited by Host>