These messages are presented on your phone or device if you are in an area where you have a limited service. If you see this message you’ll be able to make an emergency phone call (police, fire, or ambulance) by dialling 000 or 112. If you see this message you won’t be able to make any other calls or use your data.
There are a couple of reasons that you might be seeing this message:
As of August 1st 2017 the Optus 2G network has been closed. If your phone is a 2G phone (or device) or a 3G single band phone (or device) then the closure of the Optus 2G network may mean your phone no longer works, which is the case of 2G devices, or has limited coverage, which is the case with 3G single band devices. The solution here is to look at upgrading your device to a 3G dual band device or a 4G device.
Or, there could be a problem with the Optus network.