For the homephone to work, the TPG-supplied modem should be connected directly to the Arris box. You can't have it behind the Asus nor the TPLink 1750.
Your Asus and/or the TP-Link 1750 should be connected to the LAN ports of the TPG-supplied modem. Their connection type should be set to Dynamic/Automatic IP.
On a side note, it looks like you contacted our Helpdesk yesterday since the Arris box failed to get a connection. At this point, however, it's already back up and running.
Let me know if you need clarification.