After they discover the power is off in the house everyone pulls out their cellphones and sees that service is out. It is odd that a group of teens like that wouldn't have had at least one person checking their voice mail or call home to tell them they were fine after so long a camping trip and were on their way the moment they got back to where service existed.
And for that matter, their batteries should have been pretty far down without being charged. Or they should have noticed there was no service when they turned their phones back on after the trip - no one has to actually turn their phone on in the service checking scene.
Many phones can last for days after a full charge, especially when the battery isn't being drained by use.