Setting IP Phones to Use FQDN’s Instead of IP Addresses

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 affects your installation in case you wish to use FQDNs in the phone configuration. If you intend to address the PBX using IP Addresses only, and do not intend to use “Direct SIP”, then all you will need to do is enter the PBX’s Private IP Address in the SIP Domain field. The rest of this document will not apply.

Assumptions For Examples In This Article

For this document’s examples, we will assume the following:

  • PBX Private IP Address:
  • PBX Public IP Address:
  • PBX Public FQDN: (resolves to
  • PBX Private FQDN: pbx.siptesting.local (resolves to
  • Typical Direct SIP User Address:
  • We want to receive AND send Direct SIP calls

Basic PBX Configuration

  1. Since we will be sending calls to Direct SIP URIs, we must enable the “Allow calls to external SIP URIs” field in the “Settings” > “Advanced” page.
  2. Since the PBX will need to accept Direct SIP Calls in the format “”, we need to set the PBX’s SIP Domain to”. This can be set during installation, or can be adjusted after installation by going to the “Settings” > “Advanced” page.

Configuring a Phone in the PBX’s LAN (Provisioning)

3CX PhoneSystem allows you to set up phones in the LAN using its built-in provisioning mechanism. Since the provisioning mechanism sets up the phones using the PBX’s IP Addresses directly, no additional work is necessary for these phones.

Configuring a Phone in the PBX’s LAN (Manual, using FQDNs)

Here you will need to keep in mind that there is a mismatch between the SIP Domain ( and the root domain of the PBX’s Private FQDN (siptesting.local).

To correctly configure the phone, you will need to set the SIP Domain as the registrar, and the PBX’s Private FQDN as the proxy, as in the following examples:

Configuring a Remote Phone (Manual, Using FQDNs)

Configuring a Remote Phone using FQDNs will require you to set the SIP Domain as the registrar, and the PBX’s Public PQDN as the proxy, as in the following examples.