You can connect your PBX using SIP connector, or you can select a solution from the Market and connect SIP PBX using REST API.
How to connect PBX using REST API
Go to CRM > Add-ons > Telephony. Scroll down the Connection page and click View all in Telephony integrations section.
Select an app and click Install. Then follow the app developer's instruction provided in the corresponding tab.
Please contact the app developer if you have any issues with the app configuration or questions about the app installation. You can find the contact information in the Technical support section.