Your phone is not going to work at all. The T-Mobile network and the Sprint network are not the same technology, and you can not switch phones back and forth, unlocked or otherwise. You will need to find an unlocked CDMA phone for Sprint, or a used Sprint phone of the same type or similar.

Also, you said you ordered a Simcard for Sprint, but Nextel is the only part of their company with Simcards. The reason they asked for the serial number is because its CDMA, which doesn't require a Simcard.

I have the unlocked version of this phone and I recently had to change carriers, which was from t-mobile to sprint. When I had put my t-mobile sim card in the phone it worked right away. I purchased a nextel sim card and I want to put it in the unlocked 5800 but I don't know how to go about it. When I called sprint they asked me for a serial number. What do I do?, I really like this phone and I want to keep it!!!