I have seen this happen several times in the past when I have purchased a new device and swapped an old device over to another line.
The last time was slightly more complicated, in addition to moving the device to another line, they changed the number for me (I gave the phone to my goddaughters so they could have a reliable way to call mommy and daddy and of course uncle John!) and I didn't need people pestering them on one of my tertiary numbers.
In any event, it was a long drawn out process and these 2 things had to happen in order to properly configure the phone.
1. They had to verify that the proper MDN/MSID was loaded into the phone (and make the necessary changes) which they did in both your case and mine.
2. They had to reprovision the phone from their end (their server lags and it may take up to 20 minutes) for it to properly hit your account/phone/device. you will be restarting the device multiple times hoping that the forced OTA will come through properly. You may want to verify with them several times to ensure that what you see is what they see and vice versa.
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Hopefully you don't also have google voice attached to your device/account, because that brought an entire different complication to the matter. In order to get the phone fully functional (started having call in /call out problems/wouldn't update/ring/etc), they had to completely remove Google Voice from the phone (on the server end) and it could never work again (cringe) since the terms of marriage between el goog/sprint are not as tight as they once were. Obviously that wasn't much of a concern for a phone used by a 9 and 11 year old, but some of us power users love our Google Voice.
9 times out of 10 you can go to a corporate store (with a repair center, make sure it has a repair center!) and they will reprogram the device as @digiblur stated above free of charge. It is possible you have a lemon of course.
I will continue trying to find a fully stock (working) ODIN flashable rom/kernel/modem (all in one) for you. In the meantime, get a tech on the phone have them confirm your MDN/MSID settings (make changes if necessary) and once thats good have them reprovision(force OTA) the device from their end.