O2 has found itself on the wrong end of an adjudication from the Advertising Standards Authority (ASA) after Vodafone filed a complaint.
Vodafone's gripe with the recently acquired company relates to an O2 ad campaign.
O2 press adverts claimed "Best ever UK mobile network performance beating all national call success rates previously published by Oftel" within a specified period.
Vodafone filed an objection to the campaign saying call success rates were not enough to justify a claim on "best ever network performance"; that Vodafone had beaten O2's 99.2 percent call success rate; and that the ads would make people think O2 had a stronger signal than other networks.
The watchdog didn't uphold the last complaint but backed Vodafone's other grievances, saying the audit methodology on call success rates used by O2 and Vodafone made it impossible to say who could boast of being the best.
The ASA also concluded that being the best network depended on more factors than call success, including network coverage and capacity. It said: "We concluded 'Best ever UK mobile network performance' was too wide-reaching a claim to be substantiated by merely an operators' call success rate. We told O2 not to make the claim on the basis of its call success rate."
The advertising watchdog also told operators to be carefully of making claims about superiority unless they can be "unquestionably" backed up.
The spat is not the first time the pair have faced off through the ASA. O2 recently complained to the regulator about Vodafone's 'best bar none' campaign. That complaint was also upheld.