me neither, or anyone guy or girl, where I am from having something besides small things in your back pocket is uncommon in general, and a phone just sounds uncomfortable to sit on.
I do it sometimes, and a lot of my friends do it, but it depends on the outfit.
The thing about Asian clothes is that a lot of them have really small pockets, especially skinny jeans, so if you have a large phone, or a phone with a case and an attachment like a pop socket, it's incredibly uncomfortable to have in your front pocket. It's also more common if you're the type to take your phone in and out of your pocket a lot to message people or take pictures, as it's easier to take it in and out the back pocket than a tiny front pocket (some days, I've had to wrestle my phone out of my front pocket). I don't know about countries outside my own region (SEA), but I see in J-Dramas people take their phones out of their back pocket sometimes, so I assume it's the same there. It's also not particularly uncomfortable, it's pretty much like sitting on a wallet, except a bit thinner usually.
That said, that's assuming I'm wearing pretty much exactly what they're wearing, jeans and a top with no pockets. If I'm wearing a jacket or sweats, or just like... pants with better pockets, I just throw it into the front pocket, though I sometimes just throw it into my back pocket out of habit.
