Sprint's ( S) woes intensified at year-end as mobile phone subscribers fled en masse. The flailing No.3 wireless shop is expected to report that it lost more than a half million customers in the fourth quarter, say analysts and industry watchers. Unlike other players feeling squeezed by competition, Sprint can't blame a softening economy for its steepening plunge. Rival AT&T ( T) is expected to report blowout subscriber gains and No.2 Verizon Wireless -- co-owned by Verizon ( VZ) and Vodafone ( VOD) -- is likely to add another big win to its robust growth record. In its third quarter, Sprint lost 337,000 subscribers, an alarming rate given the strength of the wireless market. But Sprint's losses have been a big gain for rivals. Thanks to a strong phone lineup including Apple's ( AAPL) iPhone and a big push in its prepaid business, AT&T hooked upwards of 2.4 to 2.7 million net new users in the fourth quarter, according to research notes Thursday from Cowen and Bear Stearns. Analysts expect Verizon to post a solid 1.9 million subscriber gain. Sprint's latest disappointment will be an especially hard test of already skittish investor loyalty since the company will offer no assuring business forecast, say observers. Sprint plans to postpone financial guidance until spring, after new CEO Dan Hesse completes a sweeping strategic review.