Problem: On iPhone, low ear-speaker/ earpiece volume during phone call even when volume is set to maximum.
Caused After: Sometimes after iOS upgrade or when something falls into the ear-phone cavity.
Devices: iPhone 3G, iPhone 4, iPhone 5, iPhone 6 (all variants).
Solution: As strange as it may sound, simply suck the earpiece/ ear-speaker cavity as hard as possible.
How it works: Possibly the earpiece moves back into right position.
To Verify: Call someone
If it solves your problem, please let us know in the comments. If something else works better, please share in comments as well.