The UK Advertising Standards Authority (ASA) has upheld a complaint against EE, asking it to modify its mobile Coverage Checker after a complaint about the accuracy of its T-Mobile's coverage maps.
A customer said that EE's mobile coverage checker in the UK had incorrectly told him that he would receive excellent service in his locality.
ASA also noted that words used on the website for coverage results were not qualified, misleading users into thinking they would receive 'excellent' coverage in the postcode area shown.
In a statement, ASA said: "We acknowledged the qualification that appeared below the map, which we understood related to the map only, stated that it related to outdoor coverage only and that it was intended only as a guide due to the range of factors that affected coverage.
"However, for the reasons given, we considered the ad did not make sufficiently clear that the claims that appeared under the "coverage results" were intended as a guide and that coverage could be affected by a range of factors.
"We therefore concluded that the ad breached the Code."
In a statement to BBC, EE said that the Coverage Checker offers a prediction of its network coverage across the UK.
"In this instance, we deemed coverage to be Excellent in the postcode as confirmed by testing that found over 98% of the postcode had an excellent signal," EE said.
"We take on board the ASA's ruling and will amend the disclaimer language as necessary."
EE customers are offered a 14-day window to cancel the service if they are disappointed after signing the contract.