Throughout the course of the game, you will have the opportunity to make up to 5 friends. Each friend has certain activities that they favor over others, and through doing social activities with them, they will begin to like you. Once they like you enough, they will offer their unique services to you. To access their special abilities, press --> to open the cell phone menu, to open the phone book, scroll down to your friend's name, press , scroll to their special ability, and press again to ask them for it. If they are available to help you, they will tell you, though sometimes they may be busy, as they have their own lives as well.
Now, there is no rhyme or reason to when your friend will offer their services. Once you have reached the necessary "Like" level, sometime in the future, your friend will give you a call to offer their ability to you, so once you have the correct stats, just be patient.
This particular achievement is unlocked through befriending Roman. He is awake 24 hours a day, and when you have reached a 60% "Like" level from Roman, and will go away if you drop below 15%. See "Liberty City (5)" for advice on what activities will help raise his "Like" level the best.
You can check your progress:
Start--> Stats--> General--> Roman Like
Roman's special ability is that you can call him up and he will send a taxi to pick you up and take you anywhere, free of charge.
**Can be done after completing story mode**