Hi
Like many other members, I had the same issue with the low volume during calls and although I increased the volume on the side of the iPhone, this would also alter the volume in the sound settings and, even then, I still had problems hearing the calls.
On my iPhone 5S, I always had the volume at half way and the calling volume was fine so, I did the same on my iPhone 6 Plus but the in call volume was low and I would have thought that the volume would have been the same.
After checking around for fixes, I made a note of a few that seemed to work for other members but, before trying them out, I went into settings/sound and noticed that under 'ringers and alerts', there was a volume sound bar with a slider which, when the external volume was altered, the volume was also adjusted on the sound bar because, 'change with buttons' was enabled so, I disabled it and then I increased the eternal volume and doing so made no difference to the sound bar.
I then increased the slider on the sound bar to about two thirds with the external volume still set to the half way point and, when my next call came in, I was able to hear the caller with ease because the volume in my ear was increased to my liking and, the external volume was still set to half way and also when you launch music app. I'm really pleased now because I thought that I would have to call Apple but, I'm pleased to say that, with regard to that particular issue, I
don't need to. 🙂 I hope that this helps other members who are having the same problem?