OTOH, the estimated bytes per call truly is conservative. The 8KB/s estimate is based on G.711, and using G.726 or G.729 would yield a 2-8x reduction in storage

OTOH, the estimated bytes per call truly is conservative. The 8KB/s estimate is based on G.711, and using G.726 or G.729 would yield a 2-8x reduction in storage. So this is definitely within an order of magnitude: even if the minutes of talk time are off by a factor of 10, you could offset most of that with better codecs. (Credit: commenter petrilli on Schneier's recent post about this.)