<div>Its probably not QOS if its occuring within the same physical location. Once the call is established the RTP stream is setup between endpoints and they talk directly to one another so that takes the WAN out of the loop unless its a conference call or something that needs to haul back to the Cluster. That leaves really only 2 things as far as I can see, whatever is doing your packetization (phone and/or phone load) and the talk path. I'd probably get a tac case open becuase this really sounds like a phone load issue and in the mean time I'd start looking at layer 1+2 on the switches, and check interfaces for errors, duplex mismatches etc, I can't imagaine layer 1/2 causing a volume issue but stranger things have happened.
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