one way speech zoiper in one direction of call on android more than v5

Question

I have s4 with android v5 and calls working in and out with speech on either side however on s7 & s8 the calls only have one way speech in one direction and two way in the other. It is nothing to do with the router or isp. There must be a reason for this?

Many thanks for your help

Answer 1

Please see here.