I can't hear callers, they can hear me

I have my Iwatsu IX-5810 phone plugged into my modem. I receive calls fine. I can make calls fine. People can hear me, I cannot hear them. I’ve spoken to my ISP, the maker of the modem and the maker of the phone. Nothing fixed it. The phone itself works fine, I used it from the office last week with no issue. I’m now working remotely and this issue came to be. When I pick up the phone I hear the dial tone, same with speakerphone. It also rings perfectly normal. But when I make a call I don’t hear a ring. I can hear the caller if they press buttons on the phone (key beeps?). This is a very frustrating issue, any help would be greatly appreciated. Thank you.