Why do some short messages received by mobile phones need to be downloaded? What are all these short messages?

There are two kinds, one is MMS, because the mobile phone does not support MMS function, so if someone sends you MMS, it will be stored in Monternet, and you need to download it yourself to see it. And service information. For example, Monternet will send you a text message if there is a new system or function. There is a connection address in the short message that matches your model. You can download it when you go in. .