I wouldn't screw around with porting phone numbers, just use a separate phone number for the cell.

Virgin Mobile is the cheapest, I think. You have to add 20 dollars every 90 days to keep the phone active, but if you don't add it, the phone goes inactive without losing your balance. You now have another 60 days to add the 20 dollars and reactivate the phone (a slightly tedious procedure if you want to use the phone in a hurry, but not THAT big a deal). If you don't do THAT, then you lose the balance, not good. So if you're really cheaping out, you can add the 20 dollars every 5 months instead of every 3 months, total 50 dollars/year. Alternatively you can add a 90 dollar card which keeps the phone active continuously for a whole year. Virgin Mobile uses the Sprint network whose coverage is not the greatest, but Tracphone's coverage isn't so great either.