I am not in Nanjing. What does it mean that Nanjing people send text messages to the epidemic prevention and control headquarters?

Maybe it's because the mobile phone number is from Nanjing.

China's mobile phone numbers (China Mobile, China Unicom, China Telecom) all have a fixed regional distribution, and this region is the attribution. When you answer the phone with your smartphone, it will basically show where the phone number comes from, and this place is also where the number belongs. The attribution unit is the city, that is, when your number leaves the city where your number is located, to be precise, it leaves the network area divided by the telecom company in each city, which forms roaming.