I recently purchased an iPhone 6 and immediately noticed the phone call volume through the earpiece was much lower than my iPhone 5.
Speaker volume was more than loud enough, but the low earpiece volume - even though it was at max volume during the phone call - was making it difficult to understand the person on the other end.
A quick Google search for iphone 6 earpiece low volume brings up similar issues.
A Possible Software Fix
As mentioned by Ledsteplin on the iMore forums, a possible fix is to go to Settings > General > Accessibility > Hearing Aids and toggle Hearing Aid Mode to On. After I did this and proceeded to make a phone call, the volume was only at half even though I remember setting the volume to max during the previous call. I increased the volume to max and noticed a significant difference.
I also noticed when you toggle Hearing Aid Mode to On you can no longer toggle Phone Noise Cancellation to On (which is set to On by default in iOS 8). I have a feeling the Phone Noise Cancellation setting is the actual culprit, and the actual fix is to simply toggle it Off, make a phone call, and set your phone call volume to max.
A Possible Physical Fix
If the software fix above does not work for you, you may simply need to clean the earpiece. Judging from the number of comments for whom this has worked for, this might be the actual fix.
Disclaimer: Be careful when cleaning your iPhone. I am not, nor is anyone else, responsible for potential damage to your iPhone.
Thanks to Holly Cryer's comment, she has provided a method to clean the earpiece. What follows is a copy of that comment:
After reading this site and what everyone was saying I decided to inspect my phone closer and used my fingernail to gently rub it across the ear piece, (if you're like me your phone goes everywhere, in your pocket, on the ground, tables, chairs purses... You name it, your phone has probably been there.) as I gently scratched at the ear piece I noticed dirt or grime was coming out of the speaker onto the phone I decided to grab a dry q-tip and rub the ear piece vigorously to which end the q-tip was covered in "NASTY" I then took a little rubbing alcohol and held my phone face down (screen toward the ground) as to not get it in my phone and lightly got some on the q-tip I then lightly rubbed the q-tip across the earpiece and to my surprise, my phone works perfectly now, with out having to have it on hearing aid mode.
Optional: Use a can of compressed air at a 45 degree angle of the earpiece to remove residual dirt and/or q-tip fibers.