From the 3CX Dashboard, select the "SIP Trunks" tab.
Click on the "+ Add SIP Trunk" button.
Select Country: US
Select Provider in your Country: Voxtelesys
Main Trunk No: Your Vox Number
Enter name for Trunk: Voxtelesys IP
Registrar/Server/Gateway Hostname or IP: IP From Voxtelesys
Number of SIM Calls: Call limit with Voxtelesys
Type of Authentication: Do not require - IP Based
Authentication ID: Leave Blank
Authentication Password: Leave Blank
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.