I want to lock my iphone its been stolen

Lost Mode lets you lock your iPhone, iPad, iPod touch, Mac, or Apple Watch so that others can’t access your personal information. You can turn on Lost Mode in Find My iPhone on iCloud.com.

This assumes that you have set up Find My iPhone and the service is enabled.

Use Lost Mode in Find My iPhone on iCloud.com - Apple Support

Just to be clear … your iPhone is already VERY secure and “locked” by it’s normal Passcode. (assuming that you have a truly secret, not easily-guessed passcode)

Lost mode does a few extra things - disables Apple Pay for one - but also allows you to add a screen message allowing an honest finder to return the phone to you.

FWIW, it DOES work too - left my phone in a Las Vegas taxi early evening; had it back in my possession within two hours.

Next … if (when) contacted NEVER give your passcode, AppleID, or password to ANYBODY or enter it into ANY linked web page.

It’s a scam - often appearing VERY legit and sophisticated - by someone trying to break into your phone and/or steal your AppleID identity.

Hello ~ Take a look here:

If your iPhone, iPad, or iPod touch is lost or stolen - Apple ...

~Katana-San~