Adjusting the volume is somewhat difficult, especially if you are hearing impaired. Unfortunately, I could not find a way to accomplish what you are trying to do. maybe someone else might be able to assist you with your issue. Sorry.
I have it set up the Comcast Xfinity. So I am not sure. I think it Century Link does also. It is called the Voice and runs through coaxial outlet. Sorry I can’t be of more help.
Ask at best buy they probably have one compatible. Otherwise if there's a help number in your manual call AT&T and you should be able to buy it through them.