You can do an AOL without paying an ETF. You won't get a new phone your contract will stay the same. If you started a contract in Oct 2011 you will eligible for a discount upgrade in June 2013.
If you really want to cancel your line then you wouldn't do an AOL you'd just have to sign up as a new customer after you cancelled the line since your name isn't on her bill. If you have a smartphone the ETF is $350 minus $10 a month for every month you have already finished on the contract. So that would be roughly $230. If you have a basic phone it would be $175 -$5 per month. Or about $105. If you went this route I'm not sure you could keep your same number.