A valley probably isn't a higher section of the track/s, especially when they are talking about a route from Licola to Wonnangatta to Dargo--quite a distance.
Here's some discussion I found; the question was about mobile coverage on the Australian Alps Walking Track with particular reference to Mt Speculation. 2014, however; might be better now.
--Hi Smokey - I can confirm that you can usually get phone reception on the Crosscut saw, Mt Spec, and the Viking.
--On top of most of the big Hills from baw baw to bogong you get reception to varying degrees. Usually if you can see a ski resort or the Plains off the divide you'll get reception.
--From Mt McDonald round to Spec is all within line of sight to Mt Buller, and you can get a signal even on 2G (which is limited to about 16 km from the base station). With 3G, the distances are much further, so long as you have line-of-sight, so many high peaks will still have coverage even if a considerable distance away from the nearest base station. Check your network provider's website for coverage maps, although pretty much all the networks have base stations at the major ski resorts. Baw Baw, Buller, Hotham, Falls, Thredbo, Mt Selwyn all provide coverage for a decent chunk of the trail. The biggest dark zone is probably Mt Wills round to Thredbo, although it's worth pulling out your phone and giving it a shot on Johnnies Top and Mt Misery.
Definitely worth taking a phone (although should be on airplane mode or turned off most of the time), as it can really chew the battery hunting for a (non-existent) signal when down in a valley. Having a phone will allow you to check weather forecasts (take a look at the Bureau of Meterology Australian Alps weather), and in an emergency, a phone (provided you have signal) can provide more information than an EPIRB.
Australian Alps Walking Track