There have been post were people have used it successfully as a backup for the NBN modem. It can also be used as router with the C6300’s WAN port connected to the Ethernet port of the NBN modem. To do this the WAN port has to be enabled in the C6300. It could only be used for internet as there is no VOIP function in the modem.
If you stay with Telstra when you transfer to NBN you should receive a free NBN compatible Telstra modem with 4G backup and VOIP built in. NBN CO will supply a free NBN HFC modem.
The C6300 modem will not be suitable for a FTTN connection.