In many supernatual situations it is common for technology to simply not work. This electromagnetic interference disrupts nearby electronics with their supernatural presence.
Furthermore it is not uncommon for supernatural creatures to use or manipulate devices. They can make phone calls, alter screen displays, drain batteries, make lights dim or just kill a signal entirely.
Once the players are out of the area of effect (i.e.: room, immediate area, etc) the effects subside and return to normal.