SPH-L720T stuck on Global??

Hello everyone. I recently bought a few Galaxy S4's to resell. I kept and unlocked one, then sold the others on ebay. One of the buyers just contacted me and told me that they cannot get the phone working on their sprint account because Sprint says the phone is stuck on global mode. What is global mode and is there any easy way I can walk them through correcting it? I dont rememebr seeing a global setting in the *#0011# menu.

Thanks in advance.

Unlock Samsung Galaxy s4 Sprint Sph-L720T Invalid SIM

I have a Galaxy S4 SPH- l720T model and need to unlock it .
I read a post that this model can not be unlocked(xda in 2013).
Does anyone have this model and managed to unlock it?
Here are some model specifications :
Galaxy S4
Model SPH- L720t > Triband<
Sprint
Android Version: 5.0.1
Baseband version: L720TVPUCOK3

Sorry my Google Translate english.