I suspect the most major culprits are the cheap tiny microphones, which on a flip-phone or bluetooth headset are alongside the face instead of in front of the mouth, and the cheap tiny earpiece speakers.
My cellphone is audiophile quality;
- it includes a battery (just like the Merlin BAM)
- I can feel the vibrations in my waist when someone calls (amazing LF response)
- it produces a beautifully lifelike image (at the push of button)
- it glows a really cool blue color in the dark (great at concerts)
- it has a volume control (and it is always either to loud or to low)
- it has a forward and rear radiating tweeter (for better ambience hit the "speaker" button)
- it came with an extremely expensive power cable (costs nearly as much as a completely new phone to replace this! yikes!)
- it does the most amazing audiophile disappearing act (everytime I have left it somewhere it is gone by the time I go back to look for it)
- the surcharges on my monthly bill look just like the multiple options on most audio equipment
- like any high quality audiophile system you must leave it ON at ALL times ready for use (in case she who must be obeyed calls!)
- my provider is always trying to sell me an upgrade!