T41P: Call Park Problem

[SFA1492]

After a few more tests I figured out, that it is probably not a firmware bug.
The phone is simply too fast closing the connection after sending the call park code.
Like I mentioned above my PBX returns a positive feedback tone after about a second or so and than hangs up, when I park a call by pressing "Transfer" and enter the park code manually.
If I go the manual way and hang up before my PBX returns the feedback tone I get exactly the same behaviour when I use the predefined park button.

So the question at hand is: Is there a possibility (like adding a character to the call park code) to make the phone not to hang up or at least to delay the hang up, so the PBX has time to do what it has to do to park the call?