T4x Call Transfer using Line-Key or Phone-book

[U]

Hi

On both, T48S and T42G I manage to transfer calls, as long as I enter the target-number on the key-pad.

How ever, I want to use the phone-book or the line-keys (Speed Dial). This does not work. A call cannot be transfered.
As soon as the line-key is pressed, the first call goes on hold. The second call is established. I can now toggle between the two calls, however, I CANNOT link the two together.

What am I missing?
Can this be achieved anyway?

Thank you

Firmware on the T42G 29.81.0.110