The unlocked iPhone 6 plus can be used on any Carrier
Unlocked from Apple
Ah then yes you will have the radios but Sprint will most likely not allow you onto the network still. Sprint has to approve all devices on their network (unlike GSM where you just need a SIM) and they usually only allow "Sprint Branded Devices" (a.k.a devices you bought from them).
You can try but I don't think it will work (see below).
They have that promo going on right now as well (Verizon that is). $100 for 15 GB.
Is there a link you can send me for this promo???
That's tempting. But AT&T has rollover data.
That is correct. Verizon never will (according to their spokesperson).