From the 3CX Dashboard, select the "SIP Trunks" tab.
Click on the "+ Add SIP Trunk" button.
You will now need to match your inbound rules on the IP trunk.
Guide for Inbound Rules: https://voxtelesys.com/tutorial/3cx-inbound-rules
Guide for bulk Inbound Rules: https://voxtelesys.com/tutorial/add-bulk-did-numbers-3cx-pbx
You will now need to add the IP Trunk as a failover on the outbound rules.