If you do get a China Mobile plan, be sure to write down your service passcode (服务密码). This is delivered to you as a six digit code in the flurry of text messages you get when opening a new account. Resetting it later is possible but pretty annoying, perhaps not possible to do over the app if you don't have a shenfenzheng.

To reset it, you can text 10086 "MMXG 123456 567890", where 123456 is the old service passcode and 567890 is the new service code. I would guess that stands for 密码修改. If you are outside of China when doing this, you can use +86 138 0010 0186 (this might be Beijing specific and the 138 would be different depending on province).