Solely competing on price in the VoIP space is not a particularly attractive business model. Another provider has gone public slashing pennies off PSTN termination (even to free calling in many cases). It will be interesting to see how long, and if, these rates last:
Same folks behind VoipBuster, VoipStunt and others. They have been around for a while...
They aren't even particularly cheap, except for the free ones, which i'm sure have hidden limits. I've seen plenty of providers who sell to consumers at %30 less than those rates.