T-Mobile spent five years trying to get rid of the exact spectrum Elon just called "the last critical piece" of complete phone coverage in America. This 800 MHz block is Sprint's old Nextel spectrum, the stuff that powered push-to-talk walkie-talkie phones in 2004. The DOJ forced T-Mobile to divest it as a condition of the $26 billion Sprint merger. Dish had first rights at $3.59 billion and walked away because it couldn't raise the money. T-Mobile finally offloaded it to an investment firm…