There are two function keys directly below the display, the second one of these should have a small number 2 above it on the display (when the second account is active and registered).
Press this function key and it will prompt you to dial a number using account 2.
Dialling a number without pressing this key first will send it over the 'default' account 1.
Please see the screenshot below to explain the above:
Alternatively you can select one line as the default send account - so that your preferred line is the default for outgoing calls - by doing the following:
1. You will need to login to the Gigaset Desk phone web interface - click here to see how
2. At the Gigaset web interface login page please enter in lower case ‘admin’
3. Go to Settings > Telephony > Connections
4. That page will list the phones lines that are linked to your desk phone
5. Next to each line there is a tick box that says "Default Send Account"
6. Just tick that box for your preferred line and click Save at the bottom of the screen
7. Please make sure that you logoff to exit the Gigaset web interface.