BBC News Published image captionPeople rated their mobile operator on network reliability, customer service and value for money The UK's four biggest mobile providers are being outshone by their smaller rivals, according to an annual survey from the Which? consumer group. O2, EE, Vodafone and Three were outperformed on customer service, value for money and network reliability by Tesco Mobile, Giffgaff and Sky Mobile. The trio use O2, which scored the highest out of the big four operators. Three scored lowest, with its customers the most likely to experience network outages. The four main operators serve nearly nine out of 10 customers in the UK.