The diner keeps pouring me the same cup

I'm in a diner that's also a waiting room for something I never find out. Every booth has one glass of water on it, and the waitress keeps refilling the same cup of cold coffee no matter how much I drink. There's a payphone on the wall that's been ringing the whole time, and I'm pretty sure it's for me — but I keep telling myself I'll pick it up when I'm done here.