They're line of sight, but it'd be worth a shot to know where you can get signal and where you can't. At least you're better off for part of the time.
One of the funniest was when Waylon dumped me (my fault) and I called him on the walkies and he said "where are you?" I answered by the food plot ( we have 8 food plots over 83 acres) and he said duh? It took him a while to find me. In the meantime Waylon was running back and forth in the field like a horse in a shooting gallery having the time of his life. When hubby finally found me Waylon walked right up to his four wheeler and I haltered him......maybe you had to be there , but it was funny.