To create a remote Calendar on the PBX, and associate it to a Google Calendar, you'll need to create a new iCal Remote calendar on the PBX, and then synch it to your Google calendar using the iCal url.
In order to obtain the appropriate url to associate your remote calendar with your Google calendar, first navigate to the Google calendars page, and right-click on the calendar with which you'd like to integrate.
Select Settings and Sharing
Once in the settings menu, scroll down until you get to the Integrate Calendar section. Then, depending on whether your calendar is public or not, copy the link for either the public iCal or secret iCal link.
Typically, this will the private link. As a technician, you will typically be guiding end users in obtaining their iCal URLs. Once they have obtained the iCal URL, have them send it to you.
The image below demonstrates the correct url to copy, redacted in purple.
Once you have an iCal URL on hand you are ready to configure a remote iCal Calendar on the PBX.
Select Advanced-> Calendars, and then click the Add Calendar button, and select the Remote iCal Calendar option.
Provide the calendar with a suitable name. If this calendar controls calls for a particular user, then it may be appropriate to name the calendar after that user.
Enter an optional description, paste the secret/private Google calendar iCal URL into the Remote URL field, and select a synchronization frequency.
The calendar can synchronise to the remote Google calendar as often as once every minute, or as seldom as once a week. The shorter the synchronisation frequency, the more regularly the PBX will check in with the calendar and pull changes.