There are a few voice quality bugs on in firmware. You can do a search for various tags in the forums and you'll come across several. I recommend upgrading to something post 8-5-3 (I believe a number of fixes were noted in that revision) and then just run it individually on a few phones to see if that clears up your issue.
If not, we can help you go from there. It could be a bug, it could also be a volume issue...sounds funny but true.
Hailey
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