Dear Daniel
Good day!
Thank you for posting in Microsoft Community. We are happy to help.
Per your description, currently there is no method to remove the used numbers showing in the list. This is a by-design limitation. However feedback has been raised and you can help to vote: Ability to delete/remove the used phone numbers in call forwarding. · Community (microsoft.com) the community is monitored by our product team, to help better user experience through feedback given.
Your understanding and cooperation is highly appreciated.
Sincerely,
Momo | Microsoft Community Moderator
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