Any writers want to help?

So, I've got an outline for a short story. I will probably make a short film on it some other time once the story is finished, but anyway, can anybody give me an idea on how to continue this story, we get dropped right into the middle of an ongoing sequence:

At night a phone rings in a phone box. A man answers it and brings it up to his face. On the other end of the line is a mysterious caller whom the man does not know. The mysterious caller asks the man a few questions. The man in the phone box is rather panicky and seems to fear the person on the other end of the line. The mysterious caller then asks the man in the phone box "What did it feel like when you done it? Did you have any regrets?" The man doesn't reply and there is a flash back to the man entering a store. He is standing in an empty isle and then a shop clerk comes up to him and asks if he is ok to which he eventually replies "No". The man tells the clerk to leave him alone. He walks to his car where he opens the boot and begins loading the shopping. Then, something catches his eye, there is something under one of the bags. He pushes the bags away and pulls out a small piece of paper. He closes the boot and sits in the drivers seat of the car. He looks at the piece of paper. There is a sequence of numbers on it. He looks up with a shocked expression. We cut back to the phone box. He mysterious caller is still on the phone. The man lifts his hand up, he has the piece of paper in his hand.

Now I know what I just wrote isn't detailed but does anyone have any ideas as to how I should continue and then finish the story off? I'd like a good plot twist but I've been trying for over a week now to come up with something and I'm just drawing a blank!

ANY help would be massively appreciated!

Thanks!