As far as I know, the operation of IP telephones (foreign telephones) is only possible with the latest Internet Box Standard.
To do this, you must assign a fixed IP to the telephone and switch to expert mode on the Internet Box and click on the button Add new access data under the heading Telephone local IP access data. Then select the fixed IP on the telephone, assign call numbers and select any user name and password. (These must be used on the telephone afterwards).

The phone does not offer an interface for this, possibly only with a lot of tinkering. I recommend a phone from the Gigaset series, they all offer this function.