First, navigate to the campaign where you'd like to connect the CallRail integration, then click "Integrations" from the menu on the left side of the screen.

Next, either search CallRail in the upper right search box, or click the Phone category, then click CallRail.

Then click Connect Account.

In the next window that appears, enter the CallRail API key for the customer account that you would like to connect, then click "Save". This article from the CallRail Knowledgebase provides details on where to find your API key within their interface.

The integration will then be complete, and your client's CallRail metrics will then be imported and displayed within their campaign.