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Quoted from Wikipedia:
Yoigo is the fourth mobile phone operator with a network of its own in Spain. They operate a UMTS/3G network, which at their launch in December 2006 had very limited UMTS (3G) coverage even in some of the largest cities. In places where there is no UMTS coverage, Yoigo users switch to national roaming using movistar‘s both GSM and 3G networks.
Originally the carrier was to be called Xfera, a name reflected in its nowadays registered name of Xfera Móviles, SA. It was formed in 2000 as a consortium to compete for a new UMTS license in Spain, which finally won out. At the time, its main shareholders were French company Vivendi, the Spanish building company Grupo ACS and Scandinavian cell phone carrier Sonera (now TeliaSonera).