Hello
The criterion for choosing this converter is the type of signal dialling. If the telephone is equipped with pulse dialling (old telephone), this converter can convert it to VOIP (new). Unfortunately, German plugs are mounted on the adapter. So you have to change the plug on the phone to a German plug. You can find instructions on the Internet.

Better to change the telephone cable or buy a direct adapter, which would probably also work and connect the converter to it.

Best to change the connection cable

Wirewin RJ11