Hi Sebio,
The additional cost of minutes depends on the service provider you choose. Phone.com charges a flat 3.9 cents/minute. For Grasshopper it’s 6 cents/minute.
So in other words, if you go over your monthly allotment you just get charged the standard per-minute rate. Your service wouldn’t get shut off, nor would you be charged some additional overage fee.
Hope this helps,
Jeremy