Sorry, I don't know what you mean by "we don't have any phones that are not being used, can't do transfer?"
If you browse to System Properties -> System Feature Settings -> Class of Service Options, you'll find the complete list of Classes of Service.
You say that 15 is the default that all your phones are set to. You'll want to copy that COS to an empty one. To perform the copy, browse through the pages until you find a COS that doesn't have a comment; remember this COS number. Then, go back and click on COS 15. Press the copy button. Change the comment to something you'll understand, and enter the empty COS number that you previously found to be empty. Before you press the "Save" button, edit the Call Forward No Answer and No Answer Recall timers. Now press save.
Now, go to Users and devices, and change the phone to the newly defined COS.