It all depends the continent, region, country and the carrier's network technology. In Europe, Middle East, Africa, Australia, Oceania (and most of Asia), GSM-900 and GSM-1800 are the most used frequency bands. The companies operate within these frequencies. The USA use another frequency and that's why most phones don't work well in Europe or in one of the countries found in those continents.
Most likely, the user will use the GSM-900 band for a single call in the network, but that also depends entirely on his phone.
