Maybe - but in many countries it isn't. In my European country, calling a cab means always talking to the operator (often after 2m ringing) and then waiting 10-30m. Around 10% of the time it never comes, so you'll have to call again. When it finally does, your reward is paying an extra fee for having called.

Then there are the zones; your home is a bit outside the city? The meter will be running - at an increased rate! - before the car even arrives.