We usually say "What do you call it?" or "What is it called?" (If you're pointing out something specific, you can ask, "What is this called?" or "What do you call this?") You can hail a taxi if you standing near the street and you see a taxi passing by. You "hail" the taxi by waving or making some gesture to let the driver know that you want to get a ride. We usually only say "call a taxi" if you are using a phone to call and request a taxi to come to your location. In many places in the US, there are no taxis driving around looking for passengers, so if you want a taxi to pick you up, you will need to call a taxi company and tell them your location, and then a driver will come to you.

You can say " how do you say...." Or "what's the word for .... " Or "What's the English for...." To call a taxi means you "call" for a taxi...like book a taxi.... When you call up a cab company, you can ask to book a taxi... You say something like " I'm at [hotel/street name], could you send a taxi to this address?" To "hail" a taxi means to stop a running cab on the street and get in. You directly ask " could you take me to this place...how much for the ride....do you use a meter" . You are not booking a taxi beforehand and then getting in when you're hailing a taxi.

asking "how do you call it?" is correct but you can also use: "how do you say it in English?" sometimes it depends on the context of your conversation as well. "Call a taxi" - either calling a taxi on the street or with your phone/ apps "hail a taxi" - you are literally standing on the street waving your hand on a taxi