How to change template

[Kellermeister]

I migrated from 18 to 20 and have a couple of phone with a custom template. I want to change the template to the default template.
How to do this? I cant find the feature to actualley delete the phone and reassign it to a user.
I deleted the assignemnt of the phine inside the user, but this is not helpful, because the phone stays connected and i cant reassign it, because the "mac is already in use"
please advise

 

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Screenshots of this? There should be no issue deleting and assigning a phone providing that there are no other extensions set up with the MAC and it isnt configured on someone elses RPS server.