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.
But it was their smaller rivals, known as Mobile Virtual Network Operators (MVNOs), which were preferred by customers.
Perception and reality
GiffGaff came in first place, with 91% of customers saying they would recommend it to a friend.
Tesco Mobile and Sky Mobile came in joint second place with Smarty, a new provider which piggybacks on Three's network.
O2 came in fifth place out of 15 providers, alongside Utility Warehouse. While it had a high rating for ease of contact, customers considered it below average for value for money, with only a quarter rating it as being excellent in this area.
O2 said it was pleased with its placing but acknowledged there "was always more to do".
A spokesperson added that the positive rating that MVNOs using its network received showed that it "continues to invest significant sums into our network".
Both EE and Vodafone came in the bottom half of the rankings, performing poorly on value for money. Some 7% of their customers said they had received incorrect or unexpectedly high bills.
In response, Vodafone said the survey needed to be treated with caution. In particular, it said that suggesting an "MVNO is more reliable than the network it uses is entirely about perception and nothing to do with reality".
Three earned the lowest score of the large operators, with 8% of customers saying they experienced outages which lasted more than a day, and one in 10 complaining of unreasonable or unexpected price hikes.
A Three spokesperson said: "We don't think this is an accurate reflection of our network or customer experience. Three has some of the lowest prices and unmatched propositions on the market."
EE told the BBC that it had "award-winning customer service".
Rocio Concha, director of policy at Which?, said: "A provider should not only give you good network reliability but also value for money and customer support when you need it.
"If customers are out of contract but happy with the service they're receiving, they should try negotiating a new deal, but if all else fails it might be time to switch."
Ofcom requires all providers to notify customers when their contracts come to an end.