i have an office in the UK, they have a Polycom video unit QDX 6000. when someone is speaking the mic volume goes in and out, it does not completely cut off, the volume would just decrease to about 10 % for about 3 seconds and then increase to 90 %.
It sounds like something is being a bit overdriven by the volume settings. It is like a 'gate' that closes when the signal is a bit hotter, then, after it gets back to below that loudness thrshold, the gate opens again.
I have had good luck with adjusting that input level. Also, if there is something really noisy near the mic it can also combine with the speech and can also cause the gate to close. It is more like a 'protection' to the speakers but, at the wrong settings can cause a lot of grief.