The way the insurance works is based off a couple of things.
First: Do you already have insurance on your account?
T-Mobile will only allow you to add insurance when you purchase a brand new device from an authorized carrier but the insurance will cover any device you switch to that is T-Mobile branded with certain restrictions.
Secondly: Is the phone a T-mobile 8900 or from another carrier?
If it is not a T-Mobile Device then it will not be covered.
Thirdly: (And this one I have no written confirmation on, just verbal) The unit must be used for 30 days in a row (Meaning there must be regular use at least 30 days prior to any claim).
This is sort of a probationary period to make sure people don't try to claim an insurance exchange on a used unit that was never working on their account.
Zo