1. Some Plantronics cell phone headsets (the M175, M214 and the M114) have a hidden microphone volume control built right into the 2.5 mm plug.
This control usually has three positions labeled with circles of differing sizes
(hint: the larger circles are louder than the smaller ones).
2. Every phone with a 2.5 mm jack has a headset amplifier inside of it.
These amplifiers vary in quality. Some amplifiers give off less power than others, so if the speaker is too quiet we may be dealing with a situation where the headset needs more power than the phone can give it. You may have to try a different headset. Sometimes it's better to even try a different brand.
Recommend Sennheiser for better sound quality.