someone trying to access my stolen phone

Who alerted you? It sounds like a phishing scam→Avoid phishing emails, fake ‘virus‘ alerts, phony support calls, and other scams. Apple does not ever notify you of anything like that.

Do not respond to the “alert”; if you log in that will compromise your Apple ID and iCloud data, as well as allowing the current holder of the phone to remove Activation Lock.

If you did click a link in that notification and entered personal information change your Apple ID password IMMEDIATELY!!!!!

Always keep in mind that unless they knew you phone’s Passcode … even sophisticated criminals CAN’T get into your phone.

They probably DO have your mobile number (from the stolen SIM) AND they know you’re an Apple user so your number will likely continue to be a spear-phishing target. (some of which can be very convincing)

So proceed with extreme caution going forward.

Also activate 2FA on your AppleID if you’ve not done so already.

Two-factor authentication for Apple ID - Apple Support

The Complete Guide:

▷ Follow these important steps.

▷ Call 1-800-MY-APPLE.

▷ Learn how to file a claim.

▷ Get Support.

▷ Download the Apple Support app.