I get the same message texted to my phone all the time.e and I have the same phone as you, the ZTE Z557BL or whatever it's called. I never got the update sent to my phone either. The settings say my phone is already up to date. I called SafeLink and the lady I talked to on the phone didn't even know what message I was talking about. So I'm thinking it has to be an error or something on ZTE's side? ZTE always was a sucky phone in the first place. I prefer to use Samsung over anything else.
At 3:28 AM today I got a message from 58038 that read: 'Service Msg: Your phone requires a software update. You will receive a notification on your phone. Please click 'Install' to update. It's safe and secure.' I did not receive a notification about an update, and when I manually check, the updater tells me my phone is up to date already. Does anyone know how I can get my supposed software update? I have a ZTE Z557BL on SafeLink Wireless (TracFone) and Android 7.1.1.