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If you want a phone you could use the Telstra modem as a VOIP ATA only with its WAN port connected to a LAN port on the D7000.
The first thing you will need to do is change the LAN IP address range and LAN IP Adress of the Telstra modem so that they are not the same as the D7000. Open a WEB browser on a device connected to the Telstra modem and go to http://192.168.0.1, password is Telstra. Go to Advanced > Local Network. Suggest you change LAN IP address to 192.168.178.1 and IP address range to 192.168.178.2 - 254. See section 27 of document linked in this post for detailed instructions on the Technicolor modem. The Arcadyan modem is similar.
Connect WAN port of Telstra Modem to LAN port of D7000.
Log into D7000 and disable SIP ALG. See page 219 of D7000 manual. This will ensure incoming calls work correctly.
To prevent WiFi interference should also turn off WiFi in Telstra modem.