In order to call another iCall user you must know their username.
Once you have logged into iCall, click the "Contacts" tab.
Next at the bottom of the "Contacts" section find the section called "Add a new contact".
Next, type in your friend's name or nickname in the field that says "Name"... this does not need to be their iCall username.
Finally, choose "iCall" from the "Number" dropdown and enter their username in the field next to it.
Once you have completed adding the user, they will appear in your "My Friends" list on the "Contacts" page. Just click their name, and in the sidebar that appears click their icall username and you will be connected!