Depending on where you are and where you hike, you may have a very different idea of what a large forest looks like than some people. Unless you've really traveled to go camping and hiking, or just happen to live in a very heavily forested area, what you think of as a large forest patch and what others think of may be in entirely different leagues. And just being in the woods is only part of the issue, geography has a bigger effect than all of the trees.

I'm from the Philly area, we have a pretty big wooded park, something like 2000 acres, that is entirely within the city. It's also in a valley, so when you're in the park there's usually steep hills or even cliffs all around you. Cell service gets spotty in a lot of the park, even though there is probably no place in the park where you're more than about a mile or so from major roads and cell towers and all the other stuff you expect to find in a major city, the signal just can't get through all the dirt and rock surrounding you.

It gets even worse when you get up into the mountains, driving along a winding mountain road you can see your signal going bonkers bouncing between full bars and no bars based on what mountain is in the way of a tower at any given moment. And towers and everything else are just more spread out in general, one area I go pretty regularly to you're often driving a good half hour or so between anything you'd really recognize as being a town, without much but woods and mountains in-between.

By contrast, I've also done some hiking in the NJ pine barrens, some of the sections I've been to absolutely dwarf that park in Philly I mentioned, and are generally more remote, but they're mostly pretty flat, trees aren't great for cell signals but they're a hell of a lot better than mountains, so I can usually get pretty deep into the woods before my signal starts failing me.

I've also been to Quetico Provincial Park in Canada, which dwarfs pretty much any other forest I've personally ever been to, just an absolutely massive tract of natural area, and relatively flat at that, but it's just so big and remote that there is really no cell service to speak of.