THE CAIRN

Beware on travelling on a coach over the Pennines to Manchester when it's the fifth day after the winter equinox and the moon is waning.
Nine people who did will never forget their journey. They were scattered about the coach, everyone leaving a seat free next to them, as the coach was nowhere near full. Most had headphones in, some were listening to music, some were watching video. One woman was reading a hardback book.
The coach lumbered up a steep hill over a valley-bottom mill town, leaving the brick chimneys and a forlorn looking playground on the outskirts behind. As it reached the top, it approach a small, simple stone cairn to the side of the road, and at that point, nine phones all rang, and rang out loud. It didn't matter if headphones were plugged in or connected by bluetooth, it didn't matter than many of the phones were switched to silent, they all rang at once, and rang out loud, very loud.
The display showed a call, but no incoming number, just fragments of characters that looked as if the screen was damaged. The passengers looked at each other, and answered the call.
Most only listened to the howling, terrible screaming for a second or two in shock, before they started to jab at the end call sign on their screen, or thumb at the button on the side, and after a second or two more four of them had thrown their phones down on the floor, and were recoiling back in their seats. Nothing stopped the screaming.
Then the coach crested the hill, passed the cairn, and started down the hill. The phones all went silent. The passengers looked at one another. Those who had thrown their phones down picked them up. They looked at the phones, as if there could be a physical fault. They looked at each other. Someone laughed, a nervous, inappropriate laugh.
The driver drove on. He'd driven this route for years, and when it's the fifth day after the winter equinox and the moon is waning he always puts his phone in a metal box, stuffed with foam, and slides it to the very back of the luggage compartment for the duration of the journey.