Can not perform Phone call from Calendar

Hello,
I have issue with performing Phone call.
How do I do it: In calendar, I double-click on event, then "Link to record"
(sorry but I have CZECH language version, it might be "connect with record"),
then choose the appropirate client, click on "Connect ->" (or "Link ->"),
then double-click on linked name, and finally client detail appears.
When I try to use function "Call", then "New Call" box appears, BUT I can not
click on it, it is bleeping after each click!!!
Only to solve this I have to close Client properties (Data) window. Then I
can call, I can see the Event, but I caň not see client data.
Is it bug or am I doing something wrong please?
Many thanks David