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Hi,
Sorry to hear you are getting calls from an unknown number.
The caller has there ID hidden.
You could contact your carrier to see can they assist with this issue.
If you add all of your contacts as favourites on iPhone, turning on DND will stop these calls getting through.
Beware you won't get call notifications from anyone outside of your contacts list.
“Use Do Not Disturb on your iPhone, iPad, or iPod touch - Apple Support”