Telstra are not releasing SIP details for their Voice services over NBN.
Only thing you can really try and connecting up the 5355 when the NBN is active until Voice works on it, hook it in Via it's WAN to a LAN port on your third party modem, port forwarding the VOIP ports to it http://www.voip.com/help/residential/installation/networking/firewall_setup.aspx, and seeing if you can get incoming and outgoing calls
However this is very hit and miss and may not work