Remove-UMDialplan fails with error "There are UM users who are associated by..."

Hi,

When trying to remove a UMDialplan, e.g. "Test", it fails with error "There are UM users who are associated by their UM mailbox policy with the dial plan."

The ExchangeOnline is used for UM.

Removed all users from UM with following commands:
Disable-UMMailbox -Identity email
Set-csUser account -HostedVoicemail $false
Grant-CSHostedVoicemailPolicy -identity account -PolicyName ""

Checked if still mailboxes are related to UMDiaplan with:
Get-UMMailbox -ResultSize Unlimited | ? {$_.UMDialPlan -eq "Test"}
No results, so i suppose no UMMailboxes are related to this UMDialplan anymore

Removed UMmailboxpolicy from the UMMailbox successful with command:
Remove-UMMailboxPolicy -Identity "Test Default Policy"

Last step to remove UMDialPlan "Test" with command:
Remove-UMDialPlan -Identity "Test"
returns with error "There are UM users who are associated by their UM mailbox policy with the dial plan."

Any suggestions how to get the UMDialPlan removed?
Thanks in advance,

Patrick