I think I can answer those with the answers and local knowledge.
Modem was purchased by @daftpunkdad to replace an Aussie ISP carriers modem (DoDo) also purchasing a Grandstream ATA for Grandstream FXS ports for some form of Analogue device that is needed.
DoDo are a residential grade ISP who provides a voice port on the modem so the customer can make and receive calls in a residential environment - one of the Government requirements of all ISP’s for residential customers here in Australia.
I know DoDo are very slow at helping and in the past have caused no end of pain for residential customers in my district trying to gain assistance (they are not business oriented), maybe that is why that he has opted for buying the hardware - instead of https://support.dodo.com/modem-setup-guides/setting-your-modem#can-i-buy-dodo-modem.
I believe that DoDo will not have made the voice details readily available ergo why the issues with the ATA.
DoDo do provide the voice and internet for the customer.
For @daftpunkdad - see below and adjust your ATA accordingly.
Proxy: voip.dodo.com.au [I assume this is what DODO has provided as the proxy]
Use outbound proxy: yes [I include this because your post above implies DODO needs it.]
Outbound Proxy: voip.dodo.com.au
Display Name: your account phone number starting 61…
User ID: your account phone number starting 61…
Password: your voip password
Use Auth ID: yes
Auth ID: your account phone number starting 61…