I might be overthinking this, but I'm pretty sure rejecting the call at the end is actually the "good" ending. If you look through your inbox messages, your contact told you that they would be arriving at 10:50, and otherwise you should run. I think the message you get half-way through the game isn't actually from your contact - they've been intercepted, and someone else is in that car.
My theory is that, on the internet option on the phone, there is news about thieves stealing an art piece called, The Twins. It's the same piece that shows on the lokia screen when you boot up the game. I think that you are the person that stole the painting, and are selling it to the contact. The people trying to call them are the police.