If you are not upgrading from iSymphony 3.5.4, be sure to check the change log and upgrade notes for previous versions as well.
Asterisk V14+ Support
Due to changes in the AMI in asterisk v14, iSymphony 3.5.2 or later is required to use Asterisk v14 or later.
End of Support Announcements
Support has been dropped for Asterisk version 1.8, 10, and 12 since iSymphony 3.3.x. iSymphony will continue to officially support Asterisk version 11 and 13.
Support for Desktop Client 1.x has been dropped, Desktop Client version 2.x will be required from iSymphony Server v3.4.2 onwards. To use v1.x of the desktop client a previous version of the iSymphony Server using Module 2.0.0 or 1.x will be required.
Follow the standard upgrade instructions found in the Upgrading iSymphony document for all upgrades.