We have UC520 connected via SIP trunk to a provider. We have been experiencing bad quality when more than one line is connected to a conferencing system outside. So lets say more than one person call this conferencing system through the SIP trunk from multiple extensions. The audio when people are speaking is very choppy and robotic. As soon as all except one drop out, it becomes normal.
We have over 30 mbits/sec of bandwidth available at all time so I am not sure what could be causing this.
That indicates packed loss, even if you believe that you have the bandwidth, it can still happen. That is the naturn of the Internet, for which people still prefer POTS circuits for rock-solid service, or in some cases thye get dedicated data circuits for voice.
You can confirm if that is the case having multiple extensions. calling multiple external numbers and testing quality.
You can review UC500 configuration to optimize it, but it's unlikely that would solve the problem.
As a workaround for the conference problem, you can have a single extension call the conference service, and then have other phones conference to that call, not the external number directly.