The IRA and the baskian organisation sometimes phone/phoned in a couple of minutes before hand, they don't do this to prevent deaths, they do it to maximise deaths, and to terrorise the police.
1) when the police gets to the scene in time, they don't know where exactly the bomb is, so when they tell the crowd to head to a certain direction, it could be straight into the bomb, which is what happened in northern ireland on black friday iirc a couple of years back. Or the people will just start panic, run into the street, and get wounded by glass flying around.
2) secondly, they do it to terrorise the police, because then the cops know something will happen really soon, and they will get there too late, this can give some people serious mental problems, they know they could have prevented it, but they were too slow.
don't even think about assuming they did it to minimise deaths.