Hi there, I want to share my solution for the echo in calling bug. Iv'e tried allllllll roms with no luck, tried to change values in some files, use my phone without case...nothing has solved the problem. Today I've tried to use Revolution OS ROm V9.3.7 (no the latest one) and the vendor+firmware from:

https://drive.google.com/open?id ... 3zlV_OgrMTRzTXHc5Im

The recipe:
Reboot in twrp
Format Data
Reboot into recovery
Wipe Dalvik, cache.....
Wipe again Dalvik,cache....
Flash vendor+firmware
Flash ROM
Reboot

That doesn't fix the bug 100% but you can call with now echo just lowering the volume during the call (2 points lower)

Good luck.