Hi Georges,
Good question:)
The determining factor for using the "Answer Softkey" with the Handset On Hook is whether or not the phone model has a speakerphone. If Yes then Answer Softkey is supported, if No (like the 7911,7912,7905) the the Answer Softkey with the Handset On Hook is not supported..You will see that if the Handset is Off-Hook the Answer Softkey is displayed on an Incoming call :)
Hope this helps!
Rob