T41P: Call Park Problem

[SFA1492]

Hi,

I am having some difficulties to make "Call Park" work on my T41P.
The phone has "Call Park Mode" set to "Transfer", "Call Park" is "Enabled", "Call Park Code" is set to "*21" and "Park Retrieve Code" is set to "**21".

When I receive a call which I would like to put into a queue by pressing "Park" my display will show "Transferred" for a fraction of a second and hangs up. So far so good. But the caller gets disconnected - not so good.
If I do it manually by pressing "Transfer" followed by "*21" it takes a second, I get a positive feedback tone from my PBX, my phone hangs up and the caller gets into the queue.
It works the same way if I program a DSS Key as a Transfer-Button and put "*21" in as Value. After a second I get a positive feedback tone from my PBX, my phone hangs up and the caller gets into the queue.

Once I have a caller in my queue I can get him back by pressing a DSS Key programmed as "Park retrieve". So that at least works with my PBX.

Since the manual way works and the park procedure should the same thing - accoording to my understanding - I assume the the problem is located on the phone.
Is it possible that there is a firmware bug responsible for the behaviour. Installed version is 36.81.0.110.