I haven't read the fine print but if it is like their previous "free" offers, you are essentially doing an EIP (financing) the device and the bill credit is essentially what your phone payment would be for that 2 years. The caveat is that if you cancel the line, cancel the EIP, or leave T-Mobile then you must pay the balance off.
The other thing like with any EIP you must pay tax on the full value.
Need our help? Contact us.