I think you might be misunderstanding the feature. The phone only vibrates when you’re calling someone and the other person either picks up or drops the call. With this feature enabled, you don’t need to hold the phone to your ear or switch to speakerphone while waiting for the other side to answer—perfect for when you’re walking or in public. You can just hold your phone normally, and once it vibrates, you’ll know the call has been answered or ended.
It’s a really useful feature. I’ve actually been requesting it through the feedback app for years now, ever since I first had it on my HTC One back in 2013.