I am in AU and have terrible coverage with my iphone, as do several of my friends/colleagues. However, just as in the US it seems that some of the other network

I am in AU and have terrible coverage with my iphone, as do several of my friends/colleagues. However, just as in the US it seems that some of the other networks are much better. I wonder if there is actually some issue or incompatibility in the infrastructure that leads to some problems.