I think in most countries, it's not a problem to worry about the bandwidth usage of VoIP. But it's obviously not the case here in Australia, most Internet users have download limit or even upload limit. That's why "how much bandwidth is used" becomes an FAQ. There are many bandwidth calculators out there, but here is the statistics from my ATA.
Using G729a codec, the throughput is about 3.6MB/hr in one direction.
Using G711u codec, the throughput is about 28.8MB/hr in one direction.
In my calculation, 1MB = 1,000,000Bytes, which is correct to hard disk manufacturers, Snow Leopard and my ISP.