Official taxis in Rome are white and have an identification number written on the door (usually a name of a city and a number, for instance Lima 14), and have a taxi meter. When you call a taxi company, you will be put on hold while an operator contacts the taxi closest to you. You will then be provided with the taxi’s identification code and the approximate waiting time.
Within Rome you will probably only need a taxi at night after the metro has stopped running and the number of buses has dwindled to once an hour. Within the center during daytime it is better to either use public transport or to just walk.
It is unusual for Romans to hail taxis (I was told by one driver, who did not seem to be joking, that they are too lazy to do this) so do not be surprised if you are ignored. There are taxi stands near most of the major tourist attractions.
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