HOW TO ACTIVATE AN IPHONE IF ITS STILL ATTACHED TO PREVIOUS OWNERS CONTRACT?

My aunt gave me her iphone 6s plus because she broke it so i got it fixed, when i went to get it activated for me the guy at sprint said its still attached or connected or whatever to her contract/phone plan, therefore he couldn't activate it for me. I have NO Way to get in touch with her AT ALL, so please don't tell me to call her and tell her to cancel her plan. What ELSE can i do myself to be able to activate this phone with a new sim card and number and everything? I have the apple ID and password to the phone, because everyone in the family uses my apple id, so if that helps or has anything to do with it, i just wanted to make that clear. So, i cant get in touch with her, shes older and doesn't really know how to deal with anything so even IF i could, she probably wouldn't cancel the plan anyway, but i DO have the apple id and password if there's any way i can make the phone work through icloud or find my iphone or something, as ive read articles saying that it's possible, but i dont know if those articles are about the same situation im in. Please Help!

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