Cannot hear answering system or callers leaving message

Hi everyone, we’ve got a Vtech model CS6929-4. There is only one way to set up the answering machine, with an on or off button which is fine. The problem is when we get calls we cannot hear the answering system turn on and speak nor can we hear the caller leaving a message, the phone remains silent during the whole process, we just get notification that there’s a new message on the system when its done. How can we hear messages live when their incoming? There seems to be one option, to press speaker on the phone while someone records. We used to be able to just hear everyone without a problem. Any help is appreciated.