Answering machine icon flashes and answering machine stopped working

We have the Gigaset DX800 All in One and we have several problems with it.

The most important one is that the icon of the answering machine flashes and it says there are 2 messages but we cannot listen to them, delete them or anything. When we try to listen to them we just hear a beep and it pushes us out again. It's also not possible anymore to leave a message when people call us.

Another problem is that when people call, the number sometimes keeps on hanging in the screen but person already hanged-up the phone.

Does anyone has experience with this and knows how to solve it. The isn't any costumer support for this.