I have never had a problem just calling regular old taxis.
Every year, for new years, I go to spend the night with friends who live just off Rivadavia, in Caballito, for years now. And on several of those years, I have just walked out to Rivadavia, and flagged down a taxi within five minutes. Other times, we call in a radio taxi.
now maybe its because its not a "hip" neighborhood- but there has never been a shortage of cabs there.
I cant help it if I'm lucky- to quote Bob Dylan.