I never heard about them selling off towers but i do know they received a large amount from the fall through merger with ATT.
http://www.bloomberg.com/news/2012-...-buy-t-mobile-usa-towers-for-2-4-billion.html
They sold the rights to 7,200 of their towers for $2.4B. Not a huge deal - they can always rent back and the money they get up front helps them roll out network enhancements and such.
It just seems a little desperate, which I suppose after the failed merger and the bigger guys getting bigger while they loose 1.5 million customers in 9 months, they should be desperate.