As far as roaming goes -- they should talk to people who observed what occurred when Optus and TPG / Vodafone made a deal for rural areas and via sharing (roaming I guess,) promised increased coverage ... good idea in principle ... in my locale, Optus pulled their newish tower down and now piggy back on the already stretched TPG bandwidth and for whatever reason Optus is getting bandwidth preference. Data via TPG / vodafone which was once excellent, is now often pathetic and phone calls often drop out.

Just a note: I'm rural so it goes without saying I have plans with of all three main network providers [1] ... just in case.

[1] TPG / vodafone is via MVNO