To use the gigaset N300 A IP with the 12voip operator the following settings can be used. Note that the gigaset interface differs slightly from what is described in the manual. To access the interface type the IP address of the gigaset DECT base station in a browser or type http://www.gigaset-config.com/ (...well...).
In the page of the connections wait a bit and refresh the page to see that the connection is "Registered". The caller ID number that appears to the recipient of your calls is defined in the 12voip.com website. A degree of spoofing is unfortunately possible.