911 Operator

You'll recognize soon after playing a few games which events will cause a conflict. (Shots fired, send backup!)

Some examples are drug dealers, home invasions, sometimes fights

When your police arrive at such events, there is a short delay before the "send backup" notice happens.

To get the achievement, you need 3 police units on scene before the backup request is made.

Easiest way is to send police near the event, then send them all in at once.

This achievement is easy to obtain, but requires a bit of knowledge.

You get this achievement by sending at-least 3 police units before the "need help, send backup!" tooltip message appears. Here are a couple tips to get it.

1. You can get on both Career and Free Game.
2. If you use Free Game, pick Beijing, as the city is a grid, so it's easy to navigate and keep units near.
3. If you go with Career, pick Chicago, as the city is compact and grid like, and has many crime calls.
4. Some crimes that cause the culprit to attack are fights, gang wars, the Chicago prison escape call, sometimes car theft and bitings, and even gun raids. Avoid speeding and drinking in public cases.
5. When a crime occurs, take 3 police units and get them close to the scene, but not in it, as this will cause the alert to go off, and the achievement condition fails. Keep them from patrolling away.
6. If medical units are needed, you too must provide them, or you won't get the achievement.
7. When you have them all near the scene, put the slowest at the front of a line, and place faster units behind it. When you are ready, charge straight into the scene while trying to keep them together.
8. If 3 police units are sent in, and with medical if needed, and in before the tooltip occurs, the achievement should pop, and that 50G is yours, plus bragging rights.

For me , i got it after my second go, on Beijing. I hope i help!