"When a Killer Calls" is an unusually nasty slasher flick, with some very unpleasant and unsettling sequences. The decision was clearly made to try and cash in on the remake of "When a Stranger Calls" by pretty much putting in -- almost word for word -- the phone call sequences from that movie. They seem very forced.
Additionally, the filmmaker commits the cardinal (but all too common) sin of having the heroine's friends being repulsive jerks. So for the beginning of the film, we really like and are rooting for the babysitter (a nice believable job by Rebekah Kochan), but then she's joined by standard slasher-flick teenage friends and the mood is ruined.
The flick sort of works, but it probably a lot more unpleasant than you'll be expected, so be fore-warned.