So what then about my old mum who has one of these and some other box and a landline phone connected to it. It's her only phone. So if the box conks out she has no phone. Aren't phones considered to be an essential service?
They were an essential service with a high availability mandated by regulations. But, there was always the option to sell customers "looks like a landline phone, smells like a landline phone, works like a landline phone, but sign this piece of paper to agree its not reliable like a landline phone".
That made sense for a VOIP line running over the generic IP network, but they have taken the same position with the NBN phone lines. Several problems with that:
a) there is segregated routing on the NBN for phone services. The supplier has control of the network right up to the phone.
b) NBN connections all (residential) come with a phone number, you can't get "naked" services.
c) NBN retailers are selling locked hardware (like the OP) where the user can't configure their own system or obtain the configuration/credentials to use their own hardware.
d) exchanges/central offices used to provide battery (and diesel generator) backup for operation through power outages. The NBN network pulled that out for cost savings.
Everything there is to the convenience of the retailer/supplier, and worse for the customer. NBN, pay more for less!