Remote area, no bars. It's a horror movie all by itself!

I had an idea once for a short story, I should get on this one of these days...

Portland, Oregon has a train tunnel that is 260 feet below ground.

https://en.m.wikipedia.org/wiki/Washington_Park_station_(TriMet)

I imagine an event where the power goes out and the train is stuck in the tunnel. The passengers have no cell service and are using the lights on their phones to get around and find out what is going on.

One by one the phone batteries start to die and the lights blink out.

These are valid points. But still, I think there is little point in repeating templates just because making something worthwhile is challenging. Especially in art of any kind. I mean, it's been a mantra since mid last century at least that everything is done and yet artists of every discipline keep pushing the envelope to achieve something original.

That’s not really a hard question to answer. Cellphones batteries die. You can lose your phone or have it taken from you. It can be damaged. You could be physically isolated and have no signal. It could not work due to interference. You could be overheard. Hell even 911 call systems go down from time to time. Maybe the police are even in on it.

In pre-cellphone horror movies you still had to answer the question of “How come they don’t just start screaming for help or for someone to call the police” and if you can answer that question, the answer for the cellphone question isn’t much of a stretch beyond that.

Cell phones basically ruined horror movies.

This is an oft repeated idea. I don't really buy into it though.

If you're in immediate danger, a cellphone isn't really going to help you. Sure. Call the cops, or whatever. While you're fumbling with the phone, the killer has a chance to catch up to you. Maybe you drop the phone and have to leave it because the killer is so close. But "dropping the keys" is already an annoying cliche, so let's avoid that. You manage to call the cops. It'll take at least a minute to explain your situation to the dispatcher. Assuming they believe you (ie, you aren't dealing with a supernatural threat, which they'd either assume you're pranking or having a mental health episode) and immediately request a police response, it'll still be another 3+ minutes in an urban setting until the police arrive, or 30+ in a more rural setting (add on another minute or so if they don't believe you and you need to request that they send police anyway). If you're camping, staying at a cabin in the woods, or in some other remote setting, you're probably on your own for at least an hour.

So you've managed to call the police and know they're on their way. How long do you have to fend off the threat? It only takes Leatherface a minute or so to hack you into pieces with his chainsaw. Ghostface just slices your stomach open and is gone fifteen seconds later. Maybe you can lock yourself in your bedroom. That's not going to help if the second or third killer was already hiding in your closet. Or they can just dowse the house in gasoline and hide in an alleyway to see if you try to escape. It's a horror movie, they'll be able to evade the initial police response. If you're a primary target, they can just come for you later. Or just move on to the next target of opportunity.

If the threat is supernatural, walls are meaningless, and police are powerless.

That's not the only use for a cell phone. It can also be used as a light, but screen time kills battery pretty quickly (not sure about using the camera's flash as a light source, I'll assume that also drains the battery pretty quickly). But that light, along with either the ringtone or vibration motor can serve as a beacon if you're trying to hide from a threat.

The camera could be used for evidence. Well, see Blair Witch Project, et al. Horror already is capable of dealing with character with cameras, nothing new there.

TL;DR:

If your movie takes place after cell phones are commonplace, you need to establish some answers beforehand about “why dont they just call the cops?” and you need to establish those in a way that feels natural to the plot otherwise it will stick out to your viewers.

A decent movie can handle these issues easily (either taking a few seconds to show why the cell phone can't be used "right now", or having an opening kill where the character does manage to call the police and dies anyway.)