I am in a similar situation: I am currently on NBN with "landline" phone and I am looking at switching to TPG Home Wireless Broadband service using Vodafone 4G. In my case, I won't be able to use the landline anymore but TPG helps partly by providing a "virtual phone" with the wireless service which enables calls to the landline phone number to be forwarded to either another landline number or a national mobile phone number. However no calls can be made from the landline; to make calls you will have to have a mobile phone. What is also not clear is who pays for the calls which are forwarded from the landline to the mobile phone.
After some research I found this thread which claims that the person (that will be you and I! - not a happy situation!!) who does the forwarding pays for it!
https://forums.whirlpool.net.au/archive/1801427
