# Interconnect Two Cloud Voice Systems

When a business runs a separate phone system at each site, the calls that staff make between those sites are treated as ordinary outbound calls and add up on the phone bill. Interconnecting the two Cloud Voice systems removes that cost: once the systems are linked, a call from one office to another travels between the phone systems as an internal extension call, no matter how far apart the sites are.

This page describes a worked example so you can see how the pieces fit together before you configure it.

## Scenario

A company has its headquarters in New York and a branch office in Los Angeles. Each site runs its own Cloud Voice system. The company wants employees at either site to reach colleagues at the other by dialing an extension, with no charge for the call.

:::caution
The two systems must use separate extension number ranges. If an extension number exists on both systems, calls between them will fail. Plan your numbering so the ranges never overlap.
:::

The offices in this example use these extension ranges:

| Office | Extension range |
| --- | --- |
| Headquarters (New York) | 1000 to 1200 |
| Branch office (Los Angeles) | 2000 to 2050 |

:::tip
The simplest way to keep ranges from overlapping is to give each site its own leading digit, as in this example: headquarters uses the 1000s and the branch office uses the 2000s. That leaves room to add extensions at either site later without ever colliding.
:::

:::note
Before you begin, confirm both systems are running a current firmware version. This example was built with both systems fully updated, and the interconnection settings behave most predictably when both ends are on up-to-date software.
:::

## Network layout

The way each system reaches the network shapes how you connect them. In this example:

| Office | Network |
| --- | --- |
| Headquarters | The system is reachable from the public internet at its own domain name, for example `hq.example.com`. |
| Branch office | The system sits on a private network and cannot reach the public internet on its own. |

Because only the headquarters system has a public address, the two systems are joined by pointing the branch office at the headquarters domain name. That public address is a fully qualified domain name (FQDN): the complete, internet-reachable name of the system. The branch office system uses it to find and connect to headquarters. The diagram below shows how a call crosses between the two sites:

:::caution
The headquarters domain name must stay valid and reachable from the internet. If that address changes or the system goes offline, the branch office can no longer connect to it and inter-office calls stop working until the link is restored.
:::

![Cloud Voice, network diagram showing a headquarters system reachable at a public domain name and a branch office system on a private network connecting to it](/images/pbx/connect-two-pbxs-via-ce.png)

## Set up the connection

For the step-by-step configuration on both systems, see [Connect Two Cloud Voice Systems](/pbx/integrations/cloud-voice/connect-two-cloud-voices-via-fqdn/).