Please login to your Gigaset web interface settings  - click here to see how to do this. 

When logged into the Gigaset Base Station web interface please click on the Settings tab. 

Then click on the Telephony, Number Assignment option and make sure that 'for outgoing calls' is selected against your VoIP line as shown below:

Click on Set to save this setting.

Log off when you have finished.

Your phone should now allow you to make outgoing calls from the VoIP line you have selected.