Right now, T-Mobile and AT&T are required by law to submit blacklisted IMEIs (lost/stolen) to the IMEI Blacklist Database. Both carriers read and write to this same database and this helps prevent someone from using a stolen AT&T phone on T-Mobile and vise-versa.

Do note, MVNOs who operate on AT&T and T-Mobile are also affected by the IMEI Blacklist database, since the devices register to the actual carriers towers.

Sprint and Verizon will be required to use the same database, starting sometime close to the end of the year.

Also, the way I understand it, a device can only be put on the Blacklist Database if it has been reported lost or stolen. This should mean devices that have ETF or unpaid balances should not be on the Blacklist Database.