Hello,
In order to register your DECT phone with the Gigaset GO-Box, you first have to set up your phone, and this setting obviously depends on the brand of your handset. You will probably find the procedure described in the manufacturer's manual. Then, as soon as your handset is ready for registration, simply press the button on the front of the GO-Box (for 2-3 seconds). That's it! Normally you should receive a confirmation message on your phone.

Set the Gigaset pin if necessary, default is 0000. Search for the DECT phone in the menu under Settings 'Connect' and press OK, then you will usually see a text ..... Press the Gigabox button until the LED flashes. The handset then asks you to enter the PIN. After entering the PIN, the phone should be paired. In the web interface of the Gigabox you can make further settings, e.g. change names, load phone book, etc.
Actually, this is what the instructions say. It works for me with 5 phones. 👍😉🍀