People in the UK apparently pocket dial 999 (their 911) too often, so Emergency Call Centers automatically drop calls when no one starts talking to them. While that solution does reduce call volume, they also could have just changed the number.
Someone chose call-dropping over number-changing, hopefully because they ran an experiment and found that more people died because they couldn't remember the new number than due to call-dropping. Realistically, what do you think the odds are that they probably just didn't want to pay for the public awareness campaign to change the number?
However, they already needed an awareness campaign, as they intended to change the expected behavior from "answer call" to "answer call maybe".
And now it looks like Emergency Services has a very public kidnapping, rape, and murder of a young girl on their hands because they chose to change the system without letting everyone know. Given budget constraints, I'm sure these trade-offs and shortcuts happen constantly.
So, the next time you call 911, make sure you scream....