An
article mentioned they do
not have phones with them, so if true that means no chance they'll pick up any signal at all for 911.
I still have high hopes. It
is a huge park--larger than the entire state of Delaware, most of it densely forested and wild. The trail they were on is, IIRC, the largest backpacking trail in the park and has multiple "loops," the largest being an 88 km (54 mi) hike. Plenty of room to get lost. Lots of people camp in Algonquin, but the larger loops aren't exactly highly-trafficked. Once you're out of view of the trail you're in trouble.
However, Algonquin is full of water (best boiled, filtered or chemically purified--but worth the risk if there's no other option) and the weather is ideal right now, around 12C at night - 30C mid-day. Experienced young campers could survive quite a while. I hope that these girls are sticking together, camped out somewhere, and found soon.