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Despite ample bandwidth something is causing bottleneck in LAN and affecting VOIP

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Last Modified: 2015-08-30
Over the past 2/3 months something in our network is generating a lot of traffic and it is causing VOIP calls to sound garbly and occasional web browsing hiccups. Both the VOIP and the ISP have tested their networks and said all seems well.

Question: is there a software or service that allows me to visually understand if the issue is due to a specific computer running some sort of bandwidth-sucking software or perhaps some malware generating insane outbound traffic?

We have even received a warning page when accessing Google telling us that there is too much traffic coming from our network and asking us to prove that we are humans by filling a Capcha screen.

Our basic configuration:
100mbps / 20mbps Cable Internet, consistently testing at the advertised speed
Gateway is a CheckPoint 600 appliance, doubles as UTM and firewall, QoS function enabled
3 unmanaged switches at key points in the network (including 2 Ciscos)
About 20 users on the LAN plus some stuff on wi-fi
16 VOIP phones, not very actively used
Do not believe there are any p2p/torrent users or something like that going on
Degradation in VOIP quality happened recently and NOT as a result of any significant changes in the network

I am puzzled by this, first time in 20 years that I have encountered a network issue that stumped me.
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