I solved this issue.
The earphone volume, even for phone calls, is affected by Settings/Music/Volume Limit.
That setting is a slider labeled MAX VOLUME.
Slide it all the way to the right.
You can then hear normal volume in your ear phones during phone calls, etc.
I don't use my phone for Music, I use it for a phone. But I have to go to "Music" to fix the low volume problem.
Unbelievably bad design by Apple.
This isn't limited to the 5 & 6 iPhones. The earphones Mac Book Pro and iPad are noticeably worse than the Earbuds. The Macbook is a 2011 model that used to work well with the earphones from the iPhone 4, they no longer do, just tinny and low volume. So it appears that OSX has changed to also ensure you now have to use the later earbuds.