Set a screen lock on your phone and a PIN for the SIM card

The SIM card is that tiny card inside your phone that receives SMS verification codes. If your phone gets lost, a finder can remove the card and insert it into another phone to receive those codes, then reset all your accounts one by one. Setting a PIN on the SIM card means that whenever the card is moved to another phone, the user must enter that PIN before the phone can be used — effectively blocking that path.

No cost at all. You only need to set the screen lock and the SIM PIN once each.

Someone finding a lost phone may move its SIM to another handset, receive SMS verification codes and reset accounts. A SIM PIN requires a code when the card is used in another phone, blocking that route. In an October 2020 case described by China's Ministry of Industry and Information Technology, criminals used a stolen phone number and verification code to obtain identity details from a government app, change the mobile-service password, trick customer service into reactivating a subsequently suspended SIM, and exploit online-lending password recovery to steal funds. The ministry advised setting a SIM password and promptly suspending a lost SIM. The US Federal Communications Commission recommends a strong device password to restrict access, unauthorized charges and personal-data theft.

Find the SIM-lock setting on the phone. Factory PINs are often 1234 or 0000; after three incorrect attempts a carrier-provided PUK is required. Write the PIN down safely after setting it. The original grade is C: these sources are official warnings plus an individual case, not research measuring the money saved by a PIN. See item 4 for what to do after losing a phone.

工业和信息化部 (2020). 工信部提醒！强化个人信息保护意识. 中国政府网转载工信微报，2020-10-22. https://app.www.gov.cn/govdata/gov/202010/22/464049/article.html；US FCC. Protect Your Smart Device. https://www.fcc.gov/consumers/guides/protect-your-mobile-device