On the phone call only one side has sound, because under the current technical conditions ( Bluetooth 4.2 ), we can't synchronize two sides voice signals when on a phone call, Apple's Airpod can, but they developed a special chip to achieve this function, of course, this brings a high cost.
We are also aware of this problem, so we hope to be able to solve it in the next generation of products. So sorry our answer doesn't help you solve the problem right now.
Sincere advice if you are particularly need on this feature, you can only choose Apple Airpod. Because there doesn't seem to be any other brand that can do this right now, including worldwide top brands.