Haucke,
What does the display tell you when you press the key? No Call to Retrieve (similar to pressing the Hold button), Busy? Have you tried pressing the Answer button if you have one?
A Call Key only has a couple of states. Incoming Ringing (flashing), Answered (solid), On Hold (flashing).
Have you tried doing a Reverse Transfer from another phone while that phone is idle?
Other than that delete the phone in programming and recreate it.
Thanks,
TE