How do I report my iphone stolen

See this Apple support article—>If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

Basically, you notify your carrier, the police, and you go to https://icloud.com/find and put the phone in Lost mode.

By contacting your local police and/or asking your wireless carrier to blacklist its IMEI. Apple doesn't take reports.

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You don't report it to Apple.