I wondered this and spoke to Samsung on the online chat and the guy said that they send out a return envelope 7-10 days after you order the new phone. I pressed him on this as this contradicts the terms and conditions of the trade in offer and he said that it should say that the old phone needs to be returned 7 days after receiving the return envelope. Not sure who to believe but I'm going with the support guy and have the chat transcript as proof! Hope that helps.
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