You can make forward (forward prefix + TG prefix + external number).
If incoming call to set from outside - call will be forwarded.
If call to set internall - system will check calling set tel. facility cat. (item follow external forwarding), if follow external forwarding=1 - call will be forwarded, if follow external forwarding=0 - call will state on called internal set.
If you are tring to foward the calls from the set to external no then pls check the connection catgory 5 which will be to 0 to connection cat. 5 make it 1 and then check it has to work.