From the description of the bomb, it was not a phone call triggered. It was a very simple time bomb made using a mechanical kitchen timer. Kitchen timers are available anywhere, making tracing a purchase or former owner very difficult (especially if the bomber salvaged a perfectly functional timer from the trash, that would render it untraceable to the bomber). But cell-phones? Not so much, if they found an intact cell-phone, even one that the bomber bought in cash from some small store, they would have had a much easier time tracking who they were.