Well, gee, this is a real puzzle.
Have you experienced this issue on site your self?
AND - Some insight.
I had an 008 MLX Module in service on our system for a number of years, and all of a sudden (BUT ONLY ONCE IN A WHILE) the hand set on the MLX PHONES would not TRANSMIT. That is, it was like I had pressed a mute button. (WHICH I HAD NOT)
Replaced that 008MLX mod, and all is well.
IN all my years with this product, this had to be the weirdest thing I have ever encountered.
Could this be your issue? That is, a problem with your MLX or what ever card?