Callback is released under the GPL version 2:
You may freely redistribute and/or modify the code provided that you adhere to the licence terms.
Callback was originally derived from the FreePBX callback script. The modifications which I have made to the code are copyrighted © 2009-2020 by John Fawcett. No use may be made of the code except that specified under the GPL, unless previously agreed with the copyright owners.