I came to know about this trick. I do not have any sprint iPhone 6. So, I request anybody to test it and confirm the same
Sprint Activation (Manual):
Step 1: Write down your SIM card serial number.
Step 2: Put your SIM card in the iPhone 6.
Step 3: Press “*2″ to contact Sprint. One of the employees will contact you and activation your SIM card to be used on the iPhone 6.
STEP 4: Once your SIM has been activated, connect your iPhone 6 or iPhone 6 Plus to computer and start iTunes.
STEP 5: Now press on the restore button and it will start restoring your iPhone. Make sure you create a backup before doing this, as it will erase all your data.
STEP 6: Once the restoring process has been completed, it will show a message saying your iPhone has been successfully unlocked.
That is it folks, if you have followed the steps correctly your new iPhone 6 or iPhone 6 Plus should be unlocked and that too for free.