The HTC Titan

AT&T is expanding its Windows Phone portfolio, announcing Monday that it will add three new 4G models to its device portfolio by the end of the year. The carrier is also said that it will be upgrading all its current Windows Phones to the next mobile platform, Windows Phone 7.5 (Mango) this fall.

The new 4G phones included two new models from Samsung — the Focus S and the Focus Flash — and the HTC Titan. All will run on AT&T’s 4G HSPA+ network.

The Focus S has a 4.3-inch screen, 1.4 GHz processor, a front-facing 1.3 MP camera and a rear-facing 8MP camera. The Focus Flash has a 3.7-inch screen, a 1.4 GHz processor and a 5-megapixel front-facing camera. While AT&T hasn’t given pricing specifics, the company’s news release promises that the phone will be “light on pocketbooks.” Both will launch this fall.

The HTC phone has the largest screen size of any smartphone AT&T offers with a 4.7-inch screen, a 1.5 GHz processor and an 8MP camera with a dual LED flash.

AT&T currently offers four Windows Phones, and claims that it sells more Windows Phones than any other carrier.

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