Tonks,
There are plenty of areas with no coverage at all, especially in the valleys, but I am amazed at the times where you do find coverage.
I guess you have got to think that there is Mt Buller, Mt Hotham, and Falls Creek, so you pick up a lot from there, and then there are all the towns dotted around the fringes, Omeo in the middle.
Lots of times you can receive and send texts but not be able to make calls, but I was able to make a phone call last weekend from the top of the Sentinels near Lake Tarli Khan in Licola..... and that is in the middle of nowhere