Cable is seperate altogether from ADSL - does not use a phone line.... at least that's how I think it works.....
You can change your Telstra ADSL plan - its just gonna cost ya.... you can upgrade to 10 gig d/l a month..... $60 a month on 256 and $70 a month on 512..... unless she got the half price deal.....
you can change up and down with telstra any time! No cost at all apart from the plan cost....