Who/What Made the Phone Go Dead?

The sudden non-working phone was the main cause of the movie's plot reversal but it's never explained why it suddenly stopped working. Both the antagonist and protagonist seemed unprepared for it not working, and the phone repairman fixed it so quickly ("your phone is fine") that I'm not sure if there was ever a problem with it. It seemed like a plot contrivance needed to help bring an end to the story. Did I miss something with the phone?