See my comment about two-way. It wouldn't matter much as it's rare that both parties are speaking at the same time.

It's also possible that the compression techniques for long-term storage are vastly superior to realtime codecs. The lowest realtime voice codecs are 300-600 bits per second (they sound like shit), which is 213x compression (so an hour would be under a a meg).

http://www.wolframalpha.com/input/?i=600bps*1h*300000000

81TB a day. Again, this is assuming one hour of calls for 300 million people.

I did a quick search and found this snippet: "A telephia survey said that Americans average 13 talking hours a month – with the 18-24 age group averaging 22 hours."[1]

So that is under half an hour a day average. So, let's assume 300bps (lowest realtime voice codec I'm aware of), half hour a day, I'll stick to 300M people and we get:

http://www.wolframalpha.com/input/?i=300bps*30minutes*300000...

20TB.

So maybe I was only an order of magnitude off. Still pretty sloppy of me.

1: http://www.accuconference.com/blog/Cell-Phone-Statistics.asp...

Actually, it should probably be the logarithm of the population. The calls don't need to be recorded twice.