Two complaints were raised by a member of the public, who considered that the print advert's promise of free Sky Mobile TV and the listed monthly tariffs did not make clear that they were only for a limited time.
Following the complaint the ASA looked at the advert and added three further challenges.
The watchdog asked whether the 'free' texts were really free or part of an inclusive package, whether they could be described as free 'for life', and whether the ad made it clear that the price plans were available only via the Vodafone website or telesales number.
Vodafone argued that the asterisk next to its headline pointed readers to the small print, which explained the nature of the offer.
The mobile phone company also said that it had amended the wording of subsequent ads to be more specific.
However, the ASA found against Vodafone on all five charges, ruling that the company had breached the CAP Code for truthfulness, sales promotion rules, prices, free offers and free trials.
"We noted Vodafone's assertion that the ad made clear that the prices quoted were the discounted prices," said the ASA ruling.
"We considered, however, that because the ad did not make clear in the body copy that the prices shown were the discounted prices and not the standard non-promotional prices that would be charged after three months, customers could infer that the price for the first three months was half the price shown."
The ASA added that it "welcomed" Vodafone's amendments to its ads and told the company to make it clear in the body copy of similar ads if the prices shown were the standard price or the discounted price.
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