fataldata

I was having some similar issues described as an underwater sound or tunnel like. The problem could only be detected by the off pbx endpoint, in otherwords the VoIP caller did not notice any problems. In any case our VoIP problems were resolved by installing new Routers and Switches that support QoS.
My only advice. Make sure your network supports QoS.

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Systems In Use:
S8700 CM3.0 and S8710 CM3.1.1