If on your VOIP Phone system, whilst on a call you notice that your button has a red line on it like this .

It means you have muted the conversation between yourself and the customer (or the person on the other end of the call).
This occurs because you pressed or clicked on the button while talking to or trying to put the customer on hold.
You should not press or click on the button to access it features.  You only need to move your mouse OVER (hover) the button to access the HOLD and TRANFER CALL features.

 

IMPORTANT: If your phone hold button looks like this with a red line over the microphone, it means you have pressed the button and muted the conversation with the customer.
To unmute press the again until it looks like