Optus has called it quits on its 2G network, saying it will be shut down in April 2017. You should probably let your grandparents know about it.
The telco released a statement today saying it will re-allocate some of the 2G spectrum to make improvements on its other mobile networks and services.
While people are already on faster 3G and 4G networks, there is still a small percentage of the population that refuses to move from 2G. It's generally those from the older generations who are stubbornly sticking with their 2G mobile devices from the 1990s. (I'm pretty sure my grandpa is one of them. I should really buy him a new phone.)
For those who also know people who are still using Optus 2G mobiles, do them a favour and inform them of the network's impending demise. At least they have a couple of years to research those newfangled new "smartphones".