Blue Collar Achievement
Complete all levels of Tow Truck, Fire Truck, Ambulance, and Taxi diversions
When you enter any one of those 4 cars, you will be given the opportunity to start a diversion. In this diversion you will have to do, well, whatever you would do if you drove the car for a living. Tow cars, put out fires, revive people, and take people from place to place, respectively. You need to go to the side of a normal street (not highway or dirt road) and call the corresponding numbers on your phone to get a car delivered.
Tow Truck - This seems to be the hardest one. There is no known number to call, you just have to drive around until you happen to see one. Additionally, the diversion itself is a huge pain. Please see this post for help.
Fire Truck - 555-FIRE
Ambulance - 911
Taxi - (555) 455-8008