Complex sentence: Vodafone operates a nationwide GSM service in the 900 MHz band, but in certain areas with high demand there are additional GSM services operating in the 1800 MHz band, usually from existing 900 MHz cell sites, to provide more capacity.
Split into simpler sentences: Vodafone operates a nationwide GSM service in the 900 MHz band. In areas with high demand there are additional GSM services operating in the 1800 MHz band, usually from existing 900 MHz cell sites, to provide more capacity.