The Nokia Lumia 521 is the T-Mobile USA-bound (http://blog.gsmarena.com/nokia-lumia-521-is-heading-to-t-mobile-usa-bringing-wp8-at-an-affordable-price/) version of the Lumia 520 (http://www.gsmarena.com/nokia_lumia_520-5322.php) and is set to premiere on the carrier's retail shelves (and website) on April 24. The information comes from a leaked document allegedly belonging to T-Mobile USA.
(http://cdn.gsmarena.com/vv/newsimg/13/04/nokia-lumia-521/gsmarena_002.jpg)
Image credit: TmoNewsThe arrival of the affordable new Lumia is a perfect metaphor for the circle of life as it's poised to replace the Lumia 810 (http://www.gsmarena.com/nokia_lumia_810-5024.php), which according to the leaked document has run its course and has already, prematurely, been taken off T-Mobile USA's website. The Nokia Lumia 810 is set to be discontinued at T-Mobile USA in May.
There's no word on pricing but the international Lumia 520 is among the lowest-priced smartphones (http://www.gsmarena.com/nokia_lumia_520-review-920.php) in the entry level segment going for under $150 in some markets.
Source (http://www.tmonews.com/2013/04/lumia-521-set-to-launch-on-april-24th-as-lumia-810-hits-end-of-life/)
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