How can you unlock a Sprint iPhone 6 Plus? It was paid in full without a contract
Sprint phones CANNOT normally be unlocked for use with any other carrier in the U.S., with the exception of Ting mobile, which uses the Sprint network. Additionally, both Sprint and Verizon are on CDMA networks, so any restrictions imposed on transferring is fabricated by those companies for profit. Additionally, many new phones, including iPhones work with both CDMA and GSM, the only thing stopping you from transferring is the company.
Here is a lame excuse from an actual Sprint technician:

Betty S: Some Apple devices have an MSL code, while others do not, however, even unlocking the software using the MSL code will not enable these phones to operate on a domestic carriers’ network
Betty S: Apple phones sold for use on Sprint's network are designed with an external SIM slot and sold with a SIM card to operate on Sprint's network.
Betty S: Those devices are not enabled for use on a different domestic carrier’s network because the Sprint-specific CDMA requirements are hardcoded in the firmware of the device and downloaded for operation on Sprint’s network and specific frequencies.
Betty S: This is true even within Sprint’s Network, for instance between Sprint and Virgin Mobile. These phones, if sold through a Sprint channel were not engineered as a universal Domestic device.
Betty S: Even if these devices have MSL codes that can be unlocked, the SIM cannot be enabled for use on a domestic carriers’ network.

While the firmware being different is true, it is only that way to stop customers from leaving with their devices.