New SIM card detected in lost iPhone 16 Pro Max

It’s a spear-phishing attack, attempt to collect your phone and Apple Account credentials.

It’s NOT from Apple.

Don’t respond.

Don’t provide ANY of your info.

Delete it and move on.

DO follow the steps outlined here:

If your iPhone or iPad was stolen - Apple Support

Nope - It's NOT from Apple. Apple doesn't get involved at all in lost or stolen devices. DO NOT respond to the message unless you want to give away your account details so whoever has your phone can wipe it to either use it or sell it.

If you logged into the “Apple portal” at the provided link, your Apple Account is compromised.

Change your Apple Account password immediately.

If you think your Apple Account has been compromised - Apple Support

Apple will never send a message like this. That's how you know that this is a scam.

Ignore and delete any messages like this that you have received.

If whoever has your phone knows your Apple Account and Password, they can connect the phone to a computer and erase it, then set it up for themselves or sell it. Should we assume from your question you did enter your account and password? If yes, that was most unfortunate.

If you had secured your old SIM card with a PIN the crooks never could have sent you a message. Think on.

… or had used an eSIM.