Hi Hassan,
!good question.
From bss, the only way to measure it would be to find an indicator that counts how much time DTX was enabled during a call, per direction.
When DTX is enabled, voice is not active. The maths are quite simple. In ALU, for example, such indicator does not exist.
From core network, there might be such indicators as well, but I’m not familiar with core network at all.
the real VAD probably ranges from 30% to 40%.
Regards
pix