Default [Q] GSM D802 - no SVLTE?!

Hello,

I own a D802 for a few days now and I just have performed a LTE test - I switch the phone to auto 2G/3G/LTE, the phone is indicating 4G, I performed several SPEEDTEST.net and everything appears to be fine...excect that I cannot receive any calls...all incoming calls are forwarded to voice mail.
When I make a call the phone switch to 3G , that seems to be normal, but it doesn't go back to 4G as it should...in theory that's the flow (CSFB)

Has anyone encountered the same issue? This is how the D802 is working? Is it a phone issue?...or is it the provider?