I just picked up a new iPhone 6 Plus (64GB, Black, Verizon) from Best Buy few days ago and discovered this low phone call volume issue that everyone on this post is experiencing. I too was frustrated to find that such a basic feature of a telephone is causing such grief for many; let's face it, as smart a phone this may be it's still a phone which must provide its basic needs, which is call quality.
After having read all the posts and finding no credible fix, let alone discovering what is the actual problem, i.e., hardware or software issue, I had decided I would be returning the phone back to BB. However, this morning I discovered something which may shed some light onto this problem and a solution as well. By the way I had followed all the advices which had been posted previously to my post, including removing the film over the earpiece by way of cutting out the portion covering the earpiece, all to no avail however.
The fix, at least for me was the following: go to 'Settings' --> 'General' --> 'Accessibility'. From 'Accessibility' scroll down to 'Hearing' section to find 'Phone Noise Cancellation'. Set the PNC to 'Off" position (by default the PNC is set to 'On'). Once I did this the call volume increased dramatically, almost to the point of this becoming a non-issue. This apparently lends me to believe is a software issue because the purpose of PNC is to reduce ambient noise during phone calls when one is holding the receiver to one's ear. Therefore, once PNC is disabled the phone does not interfere with the incoming call volume.
For now, I may just keep my iP6+ because aside from this call volume issue, which is now addressed via PNC setting, is a great piece of hardware. I hope many would read my post and try this adjustment to confirm if positive results can be had like mine. Please follow up by posting your results.