In this case they're hoping the NSA kept better records than the phone provider:
"The prosecution had told defense attorneys that they were unable to obtain Brown's cellphone records from the period before September 2010 because his carrier, MetroPCS, had not held on to them."
I really hope that they tried to get the records through a court request to the right people rather then just calling the company and asking for it. I find it really unlikely that such records don't exist anymore. Phone call billing is the one thing that brings money to telcos and they're often processed and analysed. They may not be exposed to public, but not keeping a backup of them stored at least on some tapes somewhere for years is really surprising...
"The prosecution had told defense attorneys that they were unable to obtain Brown's cellphone records from the period before September 2010 because his carrier, MetroPCS, had not held on to them."