Question about used phone

Hello here is the deal I am pretty new and unfamiliar to cellphones as I would like to buy one. I want to buy the phone right now and later add it under my sister's plan (family plan) However the phone I want (Samsung SGH-D807) the price is too heavy. I am thinking of buying the phone off ebay and am unsure rather to get a locked or unlocked one? A cingular or T-Mobile one? I currently have difficulties trying to figure out if it will be compatible under my sister's service plan. Is there anyone that can explain it to me? please help - thank you

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If your sister has a cing plan and you buy a cing phone or one thats unlocked (the cing phone will be cheaper) and want to add your phone to her account (provided you don't start on of your own) you shouldn't have a prob, they'll ask you what phone you want (chose the free one because you already have one) and when you get the sim put it in your phone. Easy as pie.

First, ask your sister what carrier she has? If it is cingular, for example, in my area they offer 3 free phones with free shipping if you add a line -- so why go buy a used phone.