Brittany Murphy was originally cast as Mary Kee, but was replaced by 'Rachelle Lefevre'.
Several of the conversations recorded for the film weren't rehearsed to make each call surprising for Rachelle Lefevre.
The casting department originally had Stephen Moyer set to star as Mary's abusive husband Steven. Stephen Moyer reportedly requested of director Matthew Parkhill to be put into the role of John instead as he was afraid that his success playing a menacing character in True Blood would hurt his performance for this film if he took on such an obvious role. Matthew Parkhill reported to have had similar doubts and found Moyer to be a much better match to play John.
The original ending to the film featured a young couple being shown around the apartment after Mary has moved. The final shot focused on the telephone ringing ominously. The ending that was eventually used was actually to be the second-to-last scene in the film.
The story originally existed as a 25 minute one off television program for BBC Scotland. The cast had consisted of two actors only, and only one of them was ever on screen.
The phone calls between Rachelle Lefevre and Lorna Raver were recorded in real time.
Rachelle Lefevre politely avoided ever meeting/spending time with Lorna Raver (the voice of Rose) on set so that she could keep the mystery of Rose in her mind and keep her reactions genuine.
When Matthew Parkhill called Lorna Raver for the first time to speak to her about the possibility of her appearing in the film, he found she sounded much younger than he had expected. After voicing his concerns about the young sound of her voice, Lorna revealed her agents had warned her that she may be too old to get the part and that she should try faking a much more youthful voice. After hearing Lorna's normal aged speaking voice, Parkhill was convinced she was perfect for the role.
When Mary is on the phone, rarely is the shot of her locked on her face; director of photography went to great lengths researching shots and finding odd and interesting angles to ensure that the majority of the film she is spent on the phone would not become boring to watch.