ok here you go.
1. go to Menu
2. go to Installations
3. press Advanced Call manager
4. press Manage Lists
5. press Black list
6. press New
8. It asks you to enter either number ot the name from your contact list
9. press Done
..and this person will hear "phone is busy" for ever...

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“You become responsible forever for what you have tamed”. Antoine de Saint-Exupéry