Customers are losing faith in Verizon and AT&T, according to Brand Index, an analytics and intelligence firm based in London. Consumer surveys and interviews conducted by the company found that both telecommunications giants experienced "significant value perception" declines during the past 30 to 60 days. "Since Christmas, Verizon Wireless’ consumer value perception moved to its lowest point in at least six years, while AT&T has been declining steadily since early December to fall behind the other three in this key consumer metric," Brand Index CEO Ted Marzilli said last week. One possible reason - Verizon recently announced that unlimited data customers who use more than 200GB per month will be required to switch to limited data plans by February. If they don't, they'll lose their service. A full-length movie is usually 2GB. Bad news for Verizon and AT&T is great news for competitors T-Mobile and Sprint. Brand Index concluded that the "value perception" of each company has slightly improved or stayed the same since the Christmas season. Follow @theabigailelise on Twitter.