The Speedport ISDN Adapter is a relatively cheap VoIP-to-ISDN adapter from Deutsche Telekom. The drawback is that the adapter is "locked" to Deutsche Telekom and the user interface (web interface) is disabled.
Here is how you can access the web interface. While the adapter is already powered on, you have to press both the "Register" and "Reset" button at once for more than 20 seconds. This will temporarily enable a still limited web interface, just point your browser to the IP address of your ISDN adapter. The login password is the "device password" written on the bottom side of the case.
To fully enable the web interface you have to do a bit more. I use "curl" which is available for nearly all operating systems. You have to replace "12345678" with the device password and "192.168.1.2" with the IP address of your adapter.
curl -d "login_pwd=1&pws=12345678" "http://192.168.1.2/cgi-bin/login.cgi"
curl -d "password=12345678&debug_enable=1&uart_tx_eb=1" "http://192.168.1.2/cgi-bin/debug.cgi"
Now the web interface is fully functional and you can modify the settings, e.g. disable the TR-069 interface and enter the parameters for the VoIP account of your provider. "uart_tx_eb" enables the serial console, which offers a few debug commands. However you have to open the case to get access to the serial console.