Do you have to change your contract to add on unlimited texting?

i'm getting a phone on my grand mom's sprint service [she has 3 lines]. i want unlimited texting, but my mom says you have to change your contract. i only want unlimited for 1 phone, so do i have to change the contract? also, how much would it be for unlimited texting on one sprint line?

Source:

http://answers.yahoo.com/question/index?qid=20100115201110AA0qnjB

>Open Question: do you have to change your contract to add on unlimited texting?

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