In split DNS mode, where should the DNS server not be installed when setting up 3CX without a FQDN?

When setting up 3CX in split DNS mode, the DNS server should not be installed on the same machine as the phone system. This is because having the DNS server on the same machine as the 3CX system can create complications in the resolution of network requests, possibly leading to conflicts and resolution issues. In split DNS, the intention is to distinguish between internal and external DNS queries effectively.

By keeping the DNS server separate from the 3CX installation, you ensure that DNS queries can be handled more efficiently, and you avoid potential issues related to the server's performance or accessibility. This approach allows for better management of DNS records and configurations, ensuring that internal network devices can always resolve the 3CX services without interference from the server's workload or configurations.

The other options, including installing the DNS server on a different machine, a virtual server, or a cloud server, are valid as they help maintain a clear separation of responsibilities between the DNS and the 3CX system, facilitating better performance and reliability in communication.