Short answer: if your phone suddenly shows "No Service" out of nowhere, and people around you still have signal, treat it as a possible SIM swap attack until proven otherwise.
A SIM swap is when someone convinces your phone carrier to move your phone number onto a SIM card they control, usually by pretending to be you and answering a few security questions they've gathered from data leaks or social media. Once it's done, your phone loses service completely, and every text message and call meant for you, including one-time login codes, goes straight to them instead.
This is often step one, not the whole scam. Once someone controls your number, they can request password resets on your email and bank accounts, since many of those still rely on a text message to confirm it's really you.
The clearest early sign is a sudden, total loss of signal that doesn't match what's happening around you, other people nearby still have full bars, but your phone shows "No Service" and won't reconnect.
Don't wait it out. Using another phone, a landline, or a family member's device, call your carrier immediately and ask directly whether your number has been transferred to a new SIM. Ask them to lock your account and reverse the change. Then check your email and bank accounts for password reset requests or login alerts you didn't trigger, and change those passwords right away from a device you know is safe.
Going forward, add a PIN or passcode to your carrier account so a swap can't happen with just your name and phone number, and switch important accounts from text-message codes to an authenticator app where you can, we cover exactly how in our Two-Step Verification guide. An app-based code can't be redirected the way a text message can.
This is one trick out of dozens. The Scam-Proof Playbook covers account security, phishing, phone scams, and more, in plain language, with real examples.