When they say "Telstra branded phone for country use" I am sure it is only because Telstra has specifically sought out, or ordered the manufacture of, phones with slightly better range. Those 2 or 3 models are a bit better in the country than just a "whatever" phone from somebody else.
However, that is for true country use, and you will maybe get an extra 10km from the tower. In Mandurah anything will do, surely?
I have had a Samsung Omnia Icon for 3 yrs (with Telstra) and have full coverage for most of Perth to Albany, Perth to Gero etc with maybe 3 or 4 dropouts of 20km length on those journeys. That is not bad for a phone with no external aerial same as city folk (poor buggers) use.
What annoys me is there are no phones with a hardwired aerial plug any more. I used to have a phone I could plug into my high gain aerial on the car and have full coverage pretty much anywhere within 100km of a town and that was a big bloody area, like all the way to Esperance. Try that nowadays

Shame - cos that meant comm's on some very very remote beaches where nothign else was viable, and now it is gone. Getting worse not better is weird...