I've shared your concern with completing the phone verification with our team. One of our team members will reach out and assist you directly via a support ticket.
I am having the same issue. I was a free lancer with UpWork a few years ago but removed my account because I got hired on full time at a place near where I lived. I attempted to start a new account last week but I am getting an error code that states my phone number is still in use.
Thank you for your time!