CTI Call Centre: You can't add these users because they’re already assigned to another phone in Salesforce

We are trying to add a User to the Call Centre in an SDO, but when we select the user and try and add them, we get this error message:

You can't add these users because they’re already assigned to another phone in Salesforce

And when we try and change the Call Centre via the User profile, we get this error:

Error: You can't add a call center because users can't be assigned to both Voice and Call Center (Open CTI).

We've deleted the previous two Call Centres from the org, yet we still get the error.

Questions

1. What other ways might the User be linked to a CTI Phone in Salesforce?
2. What do we need to unlink the User so we can add them to our new Call Centre?