Is this a “I can hear it kind, with static and pops” etc? Start looking at your DSPs and wiring. Also check that you are running recent firmware on all the components (gateways, TN boards and telephones).
Or is it nice and clear, as in, purposeful mixing of the audio? If so, check out what zerozeroone was saying. It’s likely a programming issue. You can also see this stuff with a list tra station if it’s easily reproduced.