Looks like you are asking about more than Pine Valley - or are you actually referring to the Pine River near the Walls?
Peter J alerted to me to more extensive coverage on the Central Plateau in this post -
http://bushwalk.com/forum/viewtopic.php?f=3&t=2977&p=34783&hilit=phone+central+plateau#p34783His walk went through Ada and Antinomy.
We did not get near there - but we did get significant coverage in places coming down from 40 Lakes Peak to the Walls.
The basic principle is that any elevated spot which can get a view towards the SE should have coverage.
Telstra also provide a coverage map - see
http://www.telstra.com.au/mobile/networks/coverage/state.htmlOnce you bring up Tasmania you can zoom in and pan to get more detail - and the Central Highlands looks fairly good.