Telstra have a number of ways of delivering phone/ data. 3G is on the higher freq prob 1800/2100 MHz and their nextg service is the old 850 MHz system. Just have a look at a telstra coverage maps both 3G and nextg. We were in Kununurra for some years and you had 3G in the town and out at Argyle it was nextg, same for Timber creek but they had no 3G only nextg so if you were on boost you would not have phone/data there but if on telstra you would. Again look at telstra coverage maps for both systems to see what I mean.
Cheers