As an administrator, you can control call forwarding for fixed numbers after the user is removed. In the Admin portal, you go to Phone numbers > Current numbers > Select the number > Configure call forwarding > Add the number you want to forward calls to.
Please note that you can forward only fixed numbers. If you need to forward calls from a mobile number, the number needs to belong to a user.
You can only forward fixed numbers.
To forward calls from a mobile number, create user for it, and add a direct call transfer to it through the Admin portal, in Profiles > Select the user > Call to Mobile number: Treat as a call to a landline > Call to landline number: call management>connect to contact/number>save.