Fritz!Box Internal Call with Pulse Dial

After some experimenting I've found out that it seems that the Fritz!Box has a non standard way of doing internal calls using pulse dial.
After picking up the phone and hearing the signal that your phone is ready for dialing you just have to do a manual click on the switch-hook a very short time.
Now the result should be 3 beeps, then a pause and then 3 beeps again.
The Fritz!Box uses this to give the indication that internal dialing is ready.
Now the dial without **.