If you use a third party router, you will lose the ability to make calls over your landline. Telstra do not provide VOIP log in details,so you canj't use a third party ATA either. This means that you are locked in to using Telstra's hardware if you want to make land line calls. There are two ways around this however:
1) Connect the Gateway to your third party router. The set up would look like this - NTD===>Third party router===>Telstra Gateway===>home phone. From a LAN port of the third party router, connect it to the WAN port of the Gateway. Disable WiFi on the Gateway. This solution may work with compromises and issues however. In my case, I could make calls but not receive them. Others have had better success.
2) Ask Telstra to move your voice service to the UNI-V port of your NTD. This may involve a fee ($240) and you may have to have the battery backup installed as well. However, you may not have to pay this fee when you first sign up if you tell Telstra you have a back to base alarm system or medical issues. With this solution, you can use any router you want and your land line is not affected.