TL;DR: The clearest sign is your phone suddenly losing signal or showing "No Service" when it shouldn't, followed by being unable to make calls or send texts, sometimes paired with an alert from your carrier about a SIM change you didn't request. Contact your carrier immediately if you lose signal unexpectedly to check for an unauthorized SIM change, set up a PIN or passcode on your carrier account to prevent future changes, and switch two-factor authentication from SMS to an authenticator app where possible.
The clearest sign is your phone suddenly losing signal or showing "No Service" when it shouldn't, followed by being unable to make calls or send texts, sometimes paired with an alert from your carrier about a SIM change you didn't request.
What Usually Causes This
This typically results from a SIM swap where someone convinces your carrier to transfer your number to a SIM they control, usually using personal details gathered from a prior data breach or social engineering.
What to Do About It
Contact your carrier immediately if you lose signal unexpectedly to check for an unauthorized SIM change, set up a PIN or passcode on your carrier account to prevent future changes, and switch two-factor authentication from SMS to an authenticator app where possible.
If this looks similar to something else you've seen, it's worth reading How to Turn Off Ad Tracking on iPhone and Android.
This pattern shows up elsewhere too — see How to Check If a USB Drive Is Safe to Plug In.
Frequently Asked Questions
What are the real warning signs?
The clearest sign is your phone suddenly losing signal or showing "No Service" when it shouldn't, followed by being unable to make calls or send texts, sometimes paired with an alert from your carrier about a SIM change you didn't request.
What usually causes this?
This typically results from a SIM swap where someone convinces your carrier to transfer your number to a SIM they control, usually using personal details gathered from a prior data breach or social engineering.
What should I do about it?
Contact your carrier immediately if you lose signal unexpectedly to check for an unauthorized SIM change, set up a PIN or passcode on your carrier account to prevent future changes, and switch two-factor authentication from SMS to an authenticator app where possible.
Should I be worried if I only see one sign?
One sign alone (like a slower device) is usually not cause for alarm — most single symptoms have mundane explanations. Multiple signs together, especially unexpected data usage or unfamiliar apps, are worth investigating.