Earlier quoted context omitted.
I was talking about this with someone the other day… How many real terrorism threats have been preceded by the terrorist telegraphing their intentions with a phone call beforehand? My prior is that this number is essentially 0 and we should ignore bomb threats as a society.
The IRA (Irish terrorists, for Americans confused at the acronym, or maybe confused at what the IRA did) did occasionally phone warnings and occasionally the information was accurate. Code words were used to authenticate the threat.
The downside was that the vast majority of warnings were hoaxes - bomb scares were dozens of times more common than actual bombs.
The other main groups - INLA, UVF, and UFF/UDA also got in on the hoax game, but didn't often do real bombs (and didn't always give proper warnings when they did - see the UVF's Dublin & Monaghan bombings for a particularly grim example).
But real bombs were just common enough that the hoaxes from whatever source had to be taken seriously and so they caused huge amounts of disruption, probably more than anything that actually exploded.