The Caller by Chris Carter (Author)

After a tough week, Tanya Kaitlin is looking forward to a relaxing night in, but as she steps out of her shower, she hears her phone ring. The video call request comes from her best friend, Karen Ward. Tanya takes the call and the nightmare begins.
Karen is gagged and bound to a chair in her own living room. If Tanya disconnects from the call, if she looks away from the camera, he will come after her next, the deep, raspy, demonic voice at the other end of the line promises her.
As Hunter and Garcia investigate the threats, they are thrown into a rollercoaster of evil, chasing a predator who scouts the streets and social media networks for victims, taunting them with secret messages and feeding on their fear.