Hello Bogdan,
welcome back to the Polycom Community.
I suggest you download and install UC Software 4.0.11 first in order to be on a currently supported release.
You can then follow this:
Oct 7, 2011 Question: Phone unable to Dial a number when Off Hook or on 2nd Call in a Conference or Digitmap issues
Resolution: Please check => here <=
the above contains logging levels you can use to troubleshoot.
Please ensure to provide some feedback if this reply has helped you so other users can profit from your experience.
Best Regards
Steffen Baier
Polycom Global Services