Can you find the OTA package that you received? Also a system.ext4.tar(.a) from a CWM backup will be just fine too.
I just got an ota update that fixed call delay problems for good. The update is only 29mb and its not jb , as expected. But the problem is fixed. Phone feels a bit faster, but maybe that's just me.
I cant find where the fix is downloaded to share it to merge it with aurora. In greece, we were on B01, now it's B02. I remind you all that all official roms i have tried used to have the call delay problem. This is the only one that fixed it.
Konsta, would a clean nandroid be any good for you to check if you can make something out of it? If not, a rom dump or something? I wanna share it... My guess is that pretty soon it's gonna be on zte site too.
It should probably be fixed in /system/lib/hw/audio.primary.msm7627a.so and/or /system/lib/hw/audio_policy.msm7627a.so so you can try switching those libs to another stock ROMs.