It is possible to configure your phone to make and receive calls through other IP PBXes. Since a phone typically supports more than one line appearance, it is possible to e.g. register line 1 with sipX and then line 2 with a different IP PBX that manages a different domain or even a different company. In this way it is e.g. possible to register your office line at home or vice versa, provided you have an IP PBX system both in the office and at home.
After adding at least one phone to your system, navigate to the phone you'd like to add the line to, and click on the "Lines" link on the left-hand side of the page.
You should see the following UI:
Click on the "Add External Line" link and fill in the registration information for this line.
After you press "OK", you will return to the lines management page for this phone. If you need to make changes to your user registration information, you can edit the line settings but what you see for line settings will vary depending on your phone model.