I've tried the remove sim "trick" that I've read on other threads.
This allowed me to have only 3G data without the disconnections, but phone calls would not work.
Using Freedompop with the Sprint_apn fix.
Edit the build.prop with these..
SPH-D710_SPR_IC FL24 SPH-D710_SPR_ICS_Opensource_Update1.zip
For the newest build on my Sprint S4 SPH-L720 all seems to be working ... data,calls,LTE all of it.
Doing some testing….
I made a script.sh that edits build.prop to add the correct strings for CDMA.