Well, I slightly disagree; if an ad claimed "lowest price" I'd assume it was lower *or* the same as other tariffs, because if it's the same as others then they're all offering the same, lowest price. But then, I'm not a member of the general public, I guess.
On the other hand, when I read any advert from any mobile phone supplier, ISP, politician etc I do always have a little voice at the back of my head endlessly repeating the phrase "You're being lied to; they're trying to mislead you in some way..."
Sad, but true. For a given value of true.