No, a cell phone is not a detonator. A detonator is: "A small, electrically activated explosive charge that explodes a larger charge." http://www.glossary.oilfield.slb.com/Display.cfm?Term=detonator
When the news reports that cell phones were "detonators" it just shows their ignorance. The cell phones were used as triggers for the detonators.
Depending on the cell phone, it should be perfectly possible for it to be used as a timer without it ever being activated for receiving calls. The reason for using a cell phone instead of an alarm clock is that it puts out an internal electrical signal which activates the ringer. The signal
It is not obvious to me why a SIM card was necessary in the phone. All the card should do is allow the phone to make & receive calls. If I were using the phone as a timer, I would not want the phone activated to receive calls. A wrong number would lead to a serious "work accident".