A customer satisfaction survey by the consumer champion found that the UK’s biggest mobile providers have fallen short on value for money and are being outperformed by smaller rivals who piggyback on their networks.

The ‘big four’ mobile providers – O2, EE, Vodafone and Three – serve nearly nine in 10 UK customers. But in Which?’s annual customer satisfaction survey, smaller providers that use the big four’s networks managed to outdo them across the board. This includes on value for money, customer service and network reliability.

O2 scored the highest of the big four providers and earned joint fifth place out of 15 providers, alongside Utility Warehouse.

It received a high rating for ease of contact, but its customers considered it below average for value for money. Only a quarter (23%) rated it as being ‘excellent’ in this area.

However, GiffGaff, Tesco Mobile and Sky Mobile, which all piggyback on O2’s network, did better than O2 in Which? rankings. GiffGaff came in first place and earned strong ratings in several categories with 91% of customers saying they’d recommend it to a friend.

Tesco Mobile came in joint second place with Sky Mobile and Smarty. Both Sky Mobile and Tesco Mobile scored higher than O2 on value for money – an area all of the big four providers fell short on in the Which? survey.

EE and Vodafone came in the bottom half of the rankings, both performing poorly on value for money. This is perhaps unsurprising given that 7% of their customers received incorrect or unexpectedly high bills – the highest across all the providers.

Three earned the lowest score of the big four providers, with value for money and network reliability highlighted as key weak points.

Three customers were the most likely to experience network outages lasting more than a day (8%), while one in 10 (10%) experienced unexpected or unreasonable price hikes.

One Three customer responding to Which?’s survey said that a universal change in tariff had made it more expensive for them to use the provider despite having a substantial balance to use up.

Like O2, Three was beaten by a smaller operator using its network. Smarty is owned by Three and is a new entrant to Which?’s survey at joint second place.

Rocio Concha, director of policy and advocacy at Which?, said: “Our research found that the biggest mobile providers are being outshone by their smaller rivals. 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.”