I don't suppose you can debug that voice port can you?
I have seen this issue before, and it turned out that the port it was plugged into went the way of the dodo birds so TAC RMAéd the system and problem went away
But that would be the simple solution to the problem, the harder one is where you debug it and everything looks find and then you start conducting bizarre and insane tests to diag the problem, only to end back at the original proposed point of swapping the system out
You can debug it now, or you can log a support request doing that now will save you many days of headaches and you will also be doing the right thing by the customer.
Just a thought
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