During set-up you are prompted to enter the FQDN of the phone system server OR the SIP domain. This SIP domain is used if you wish to use Direct SIP calls (more info here), but it also effects your installation in case you wish to use FQDNs in the phone configuration.

When you configure the phones you must enter either the IP OR the FQDN of the phone system server in the registrar field. If you enter the FQDN in the registrar field, then the FQDN must be listed as the local SIP domain. We have illustrated this with screenshots using the configuration of a SNOM phone:

In the phone configuration of the screenshot above, the registrar has been configured as phonesystem.3cx.local. Therefore the entry in General Settings > Direct SIP calls > Local SIP domain must read 3cx.local.

In case you wish to use direct SIP - that is allow people to be called using a SIP ID - then this entry will not be convenient. You have 2 alternatives:

1. Use the IP of 3CX Phone System in the Registrar field (See example above)
   
   
   
   
2. Specify the SIP domain in the registrar field and the FQDN in the outbound proxy field. (See example above)