Bandwidth Usage

I am trying to evaluate a wireless system: $20 for 500MB/mo.

If I were to use this as a VOIP device to substitute for my VOIP phone, I am wondering how many minutes does this buy me. Better yet, how many minutes does 1 MB buy if one is using Google Voice (Jabber \ XMPP)? I have seen codec bandwidth table with a range of 2.5 to 64Kbps. By my calculations this is a range of 6.6 minutes to 0.26 minutes. At 500 MB, that would be a range from 3333 minutes to 130 minutes. I would suppose that these are optimistic estimates. Not sure what codec is used during GV calls.

That being said, I would like to measure an * call: including it’s length (time), data (bytes) in, data (bytes) out. Are there counters already put in place to measure this data?