if you consider high end hardware, say 400gb harddrives even with data back up systems etc thats less than a harddrive a day.

if were looking at 1<>2 Petabytes thats 13<>26 years worth of data.  

HOWEVER

people recoment 20kbps for mp3s of phone recordings, SO using that assumption and assuming an average phonecall length of ... 20 Min (total guess here)  thats 24000kb or 2.9MBytes per call, 87.8MB per person thats close to 21.6 PETA bytes per day, far more than was discussed here.  

Now the NSA may have much better bitrate encoding schemes for audio maybe even 1kbps or 1000 bits per second, but even then we are still talking a database the size of the LARGEST currently know database EVERY DAY