MetroPCS raised $550 million in Senior Notes, on top of $300 million announced yesterday, with the same terms.

However, the prepaid operator again only gave a vague reason for why it needs to raise money, citing the same list as yesterday – “for general corporate purposes, which could include working capital, capital expenditures, future liquidity needs, additional opportunistic spectrum acquisitions, corporate development opportunities and future technology initiatives.”

Company officials did not return messages for further explanation yesterday or today.

So what does MetroPCS really intend to do with the windfall? “I would say they build out with it… It looks like they’re raising it for capex,” said Neilsen analyst Roger Entner, head of telecommunications research.

Specifically, the funds likely would be used for market expansion into Boston and New York and to pay for spectrum won at FCC auctions last year, he noted.