Currently there is no way to create a Do not call list.
There should be a way within the Outgoing call permissions to create a custom options and link this to a dial plan. This would allow numbers to be blocked based user location. Currently you can create a dial plan with no working route to block numbers, but the problem is that it applies to everyone.
Upcoming feature Enhanced Outgoing Call Permissions will address this use case. Using this feature administrator block outgoing calls to specific number or range of numbers by configuring digit patterns.
This feature is in beta now. If your customer is interested, suggest signing up beta program
https://gobeta.webex.com/welcome/
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