When the phone rings the opto isolator fires the scr
which latches the DPDT relay providing dc power to a blasting cap.
Can basically be detonated from ANYWHERE remotely by calling the phones number.
A missed call?
Yes... that is what cell phones tell you nowadays when a call comes through on your cell if you do not answer it. Most likely the example in the picture is one cell phone that failed to set off the bomb it was attached to. That is why the phone is still intact and someone called it to set off the bomb (bomb goes off normally on the first ring) but it failed to set off the bomb.