Instead of arguing, could you please do the test?
Yes, this is a very well known issue. On a stock, rooted G4, WiFi Calling works fine in Airplane Mode. It also works in Airplane Mode on my Note 4. That is how it is SUPPOSED to work. The whole purpose of WiFi Calling is to work when you have no cellular signal.
We have figured out the issue only happens when Xposed is installed on a stock odexed ROM. If you deodex the ROM, it works fine with Xposed. The problem is deodexing LG ROMs breaks many features since some APKs cannot be deodexed, so we are waiting for rovo89 to fix it.
We have a thread about this issue:
http://forum.xda-developers.com/tmobile-g4/help/wifi-calling-errors-10n-update-t3191362
And it has been logged as an issue on rovo89's github:
https://github.com/rovo89/Xposed/issues/66
Now can you please do the test for us?