In order to get a taxi anywhere in Los Santos, you simply have to access your cell phone in game and search for the taxi number in your contacts list.
Alternately, if you don’t want to call for a taxi and you want to hail a cab that’s on the street, hold Left on the dpad then TAP Triangle/Y to get in. HOLD Triangle/Y to jack the taxi.
I hope that this short tutorial answers your questions on how to hail (call) a cab in Grand Theft Auto 5 and you will have no problems now getting where you want to!