for the new tmobile plan, if i get an iPhone 5s and i end up not paying my bill will the phone still be usable

this person wants to sell me an iPhone 5s. but when i looked up the imei it said

This device is being financed and has an outstanding balance that must be paid or it may not be able to be used on the T-Mobile network.
For more information or assistance, please call Customer Care at 1-877-453-1304." so does that mean its not usable if i buy it from him? someone help