and remember you "can't " get the same numbers, unless you are willing to break your contract port your numbers to a prepaid phones, then port them back to sprint!!
I know it looks like a lot of hassle
but if you do not really care about keeping the same numbers, sero is for you, check them out at
http://www.sprint.com/sero, and as crimekilla said just use the email
savings@sprintemi.com it will get you to the phones and the plans, and yeah by the way if you are getting your sister a new mogul just check the sero prices of the phone with a new contract I think it might be cheaper than eBay.