Dear YuriRM, thank you for the two links. Unfortunately I'm not the Tech-guy, and have not even a glimpse what can be done using those things...
After getting some experience with the latest KitKat-ROM (4.4.2) developed by LuckasFB, I can tell you all that it's useable. VoIP calls work as well as GPS, Gyro, 3D. Not sure if LuckasFB is still working on better/newer firmware, so currently it is probably the best choice.
All of the hints and links are posted "somewhere" in this thread with more than 100 pages and over 1000 entries, so let me sum it up (YuriRM already did that several times before):
First, install TRWP recovery: Flash the file inside this archive (you have to extract it)
ftp://185.86.77.191/LuckAs-kernels/A5000/LuckAs-Takee_1-twrp28-recovery.rar
as recovery-file using SPFlash-Tool.
Place this two files (without extracting them) in the phones memory or the root of your phones SD-Card (if you use one):
ftp://185.86.77.191/LuckAs-kernels/Takee_1_S1/takee_1_cwet_kk-luckasfb-20161226-024100.zip
ftp://185.86.77.191/LuckAs-kernels/Takee_1_S1/4.4.2_micro.zip
Connect phone to a charger, switch it off, and when restarting it, hold "Vol up" pressed, until TRWP-Menu shows. Use "Wipe"-Function, after that "Install" and select the two ZIP-Files, install them and reboot.
Install from play store any power manager app that can do a "hot reboot" (example: "Power Manager" by Guiseppe Montuoro)
After boot wait some time (like 10 minutes), then try to open camera. When the selfie camera is started, make a hot reboot using the power manager app. Start camera again, both cameras will work now (note: as long as "unable to open camera" or "gallery error" shows up or camera just doesn't start, hot reboot is useless).
Getting 3D-Effect (only necessary if you use SD-Card): Copy the two files "HolographyProfile" and "matrix" from the phones folder "Android" into the folder "Android" on the sd-card. The (already preinstalled) App "3D Video" contains four 3D Movies. Hint: If 3D-Effect is ghosty (doubled elements in the picture) try to tilt the phone just a few degrees to the right instead of looking in the very middle of the screen.
Rather than switching off the phone at night, only put it in airplane mode. Saves energy for the battery and you from doing a camera restart the next day...
App that makes use of the gyro effect and has appropriate videos: Install DiscoveryVR from play store. Open a video (like: Shark Storm), move the phone up/down/left/right to change the viewing angle.