After program integration with the phone I can't make calls over GSM anymore

Indeed You can, but don't know how to :)

Usually it's because of another programs for calls over Internet were installed.

In that case, while trying to make call, the first menu that You will see, offers You to choose for the call Zadarma program or another program for calls over Internet. Choose Zadarma and in it choose to call on GSM.

If You don't want to to choose in two menu, find in the first menu (which is not related to Zadarma project) check mark ┬źRemember choice┬╗.

Also you can uninstall previously installed programs.