I was thinking a VOIP codec might require somewhere between 30 - 75Kbps. I had Vonage for a number of years a long time ago and I thought they offered a couple different stream rates...these numbers stick in my head for some reason.
So I considered something in the middle around 50Kbps which would be about ~150 minutes.
Of course I didn't consider that these bit rates would probably deliver land-line or better quality -- which would far exceed typical cell phone sound quality. If we lower the bar to cell quality, it's probably realistic to get a lot more minutes as you cite.