I have a SIM800 module on my rp3 B+ that works if I use Stretch Lite. Only thing needed to do was to add the pi3-disable-bt overlay and disable serial login.
Using python and pyserial I could open /dev/ttyAMA0 and send AT commands.
I can open a serial connection to /dev/ttyAMA0 within the addon but I am not getting any response back.
Any idea what I am doing wrong?