My phone was stolen and it is under warranty. What I can do in this case?

There is no warranty coverage for a lost or stolen phone.

A lost or stolen phone is only covered by purchasing an insurance policy:

AppleCare+ with Theft and Loss

Get everything included in AppleCare+ and up to two incidents of theft or loss coverage every 12 months.3 Each incident is subject to the deductible listed below.

Theft and loss coverage requires you to have Find My enabled on your device at the time it is lost or stolen and throughout the claims process.

AppleCare Products - iPhone - Apple

If you bought theft and loss coverage:

iPhone Theft and Loss Claims - Official Apple Support

You’ve gotten good advice so far, but, as Steve Jobs used to say: One More Thing -

Beware of a common phishing scam: You may get a text or email from “Apple” or “Apple Security” (or sometimes “Police”) saying your phone has been found and to click on a link to see its location. This is NEVER from Apple or from law enforcement; it is from the thief or fence to steal your Apple ID and identity. Delete any such message without responding to it. Apple never gets involved in lost or stolen devices→Avoid phishing emails, fake ‘virus‘ alerts, phony support calls, and other scams

Click here for information; the warranty doesn't cover loss or theft.

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