Mobile network Three's claim that it had made 0800 numbers free has been banned following a complaint that customers had to pay £5 a month to take advantage of the deal.
The television ad said all 0800 numbers were free on 24-month plans and urged customers to "make the most of them".
But 11 people complained that the deal only applied to new customers or new contracts, and one customer said they understood the cost of the service had been increased to cover the 0800 calls.
Three said the supporting information relayed by both voiceover and on-screen text was unambiguous and explained that 0800 numbers were free on their new 24-month plans.
The network said it was standard within the mobile phone industry to charge for 0800 calls, but its new plan allowed customers to make the calls at no extra cost.
It said prices were set to reflect a number of factors such as inflation, network investment, market conditions, network capacity, the cost of data and handset prices and it had not raised prices to include the cost of 0800 calls.
The Advertising Standards Authority (ASA) said it understood that free calls were only available with new 24-month contracts, but agreed that the ad contained ambiguous and contradictory messages which did not make clear the extent of the commitment consumers had to make to take advantage of the free offer.
The ASA said consumers would have understood from the ad that no additional charges would be incurred, but ruled that the ad breached rules because the price of the service had been increased.
It said: "The ad must not appear again in its current form.
"We told Three to ensure the basis of any 'free' claim was clear to consumers in future."