Meanwhile I have tried different scenarios and sometimes it is possible to control call volume of the airpods via the watch and sometimes not. It seems that the airpods are one time connected to the iphone and one time directly to the watch. When switching off bluetooth on the iphone and start the call via the watch then it works. When switching bluetooth on iphone back on it works at the beginning as well, but when you listen to music in the meantime it seems that the iphone take back the bluetooth connection of the airpods then the phone volume control problem is back again. I know its hard to describe, but perhaps someone could give it a try and can write here his experiences or can tell me what I´m doing wrong. Or could this be a bug?
Below two pictures of the different phone app screens on the watch. BTW proud owner of a S5 since today But the problem is the same on my S4
But the problem is the same on my S4 
