If you've moved office you may find that the phone is signed into the wrong person. Fortunately, this is easy to do by following these two guides.
First, you need to sign out of the phone using this guide: https://gatservicedesk.freshservice.com/support/solutions/articles/50000033486-sign-out-of-yealink-voip-phone
Then, you can sign into your phone using the guide below: https://gatservicedesk.freshservice.com/support/solutions/articles/50000033477-sign-into-yealink-voip-phone
For polycom phones you can sign in or out using the guide below: https://gatservicedesk.freshservice.com/support/solutions/articles/50000052549-sign-into-or-out-of-polycom-voip-phone