Is everyone aware the 2G network is closing down soon
Telstra are shutting their 2G network down in December with Optus to soon follow in April
Vodaphone may be keeping it for a bit longer
http://www.news.com.au/technology/gadgets/mobile-phones/telstra-plans-to-kill-2g-network-in-2016-forcing-users-to-upgrade-their-handsets/news-story/5d3ca84e83744f5a2adbf73d9936305c
https://media.optus.com.au/media-releases/2015/optus-to-cease-2g-services-from-april-2017/
If you have a 2g phone you will need to replace it with a 3g or 4g phone