Ran into this today and found your post.
For me, I originally used the old command starting with "Set-CsOnlineApplicationInstance" as we had in the past but calls were not working. So I removed the TN from the Resource Account, waited about 5 minutes and then ran "get-CsOnlineApplicationInstance" to verify that resource account no longer showed a phone number.
I then had to update the PowerShell teams module using: Update-module MicrosoftTeams
Close PowerShell and reopen it. Connect-MicorsoftTeams
I then ran the new command (example)
Set-CsPhoneNumberAssignment -Identity Email address removed -PhoneNumber +15555555 -PhoneNumberType DirectRouting
Not sure, but the new command allows you to specify the "PhoneNumberType DirectRouting".
https://docs.microsoft.com/en-us/powershell/module/teams/set-csphonenumberassignment?view=teams-ps&v...
