To activate call forwarding on Jabber, click on the green symbol in the lower right-hand side of the client. (The symbol will either be a phone or a PC depending on your device and set-up)

To activate call forwarding, select the line you wish to forward to.
Head to the dropdown menu under forward calls. By default, this should be set to none (1).
Select line (2) by hovering over it with your cursor.
Enter the target number (3) under the 'new number' option.
 

This opens a separate window for entering the target number.
Please use the following syntax:

• XXXXX (RWTH number)
• External numbers with a leading 0 (00151xxxxxxxx)
• Or external numbers in E.164-Format (+49151xxxxxx)
   

If you have already saved the number once and need to set the forwarding again, you do not have to enter the number again. You can simply select the appropriate number from the menu.