When I came off the ferry in Barcelona (Catalonia), I tried and failed to get a taxi. There were twelve numbers posted on a sheet of paper, they were all either busy, or not working, or I was put on hold. Since my phone provider used to charge €1.49/minute for calls to other EU countries (this was lowered to €0.22/minute weeks before I travelled, but I was not aware) I did not want to wait on a hold for too long. We ended up walking 2 km with big suitcases to the nearest public transportation (and harbour areas at night are rarely the nicest places to be), along with another group of people in the same situation.
For hotels, car rentals, flights, and buses, there are various websites (and I suppose smartphone apps) for a meta-search of different competing alternatives. None are complete, but most are useful. Is there any such service for licensed taxis?
I prefer an option with widespread geographical coverage, but if that is not available, options that work for specific countries or cities are also valuable. Ideally, I'd just let my presence known through a smartphone application, and then whichever taxi company claims me first would come to fetch me.
As a matter of principle, I will not consider any unlicensed, unmetered, or illegal taxi operator such as one is very likely to get with such services as Uber.