Mine was sprint. Edited my prior post.
But I had a chat at Best Buy a while ago about selling my upgrade. The salesman, a Sprint specialist, said he at first was uncomfortable with a customer who was doing that. But he said his supervisor pointed out that the terms of the upgrade was to keep the plan for 2 years. If you do that, you fulfill your obligation and Sprint ( or at least this manager) has no interest in which phone you actually are using.
was it verizon? I know with them they say they will charge you full retail if you don't activate the phone.
Around here we DROR, so it may not be so. Although I wouldn't want to be the one charged. Perhaps HUCA. "If I get a new phone with an upgrade, can I save it as a backup phone and not activate it now?"