Answer Best answer as chosen by user The_Sandman224
As has been pointed out, unless you are an authorized user on her account, you have no right to make any changes w/o her knowing about it.
The only way would be for you to either buy the phone outright, paying the no commitment price ($$), or 'gifting' a T-Mobile handset you obtained through your (separate) account, gaining the subsidized price if you are currently eligible for a handset upgrade. That's about it.
FWIW, I've encountered multiple offers to buy brand new S3s for $10-50 this week while shopping in brick/mortar stores. Might be a cheaper way for you to upgrade your mom to a smartphone. The stores really want to move these products now since sales are generally flat per se.
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