Specific issues can be located in the detailed release notes in the wiki at: http://wiki.sipxcom.org/display/sipXcom/sipXcom+18.04
A new ISO is not available yet for 18.04. You can get the 17.10 ISO and run the upgrade listed below ISO's are available at: http://download.sipxcom.org/pub/sipXecs/ISO/
The ISO's are built on a CentOS 6.7 base so please yum update the system after installation.
Upgrade from previous versions