Go to your 3CX Admin Page and login. After login go to SIP Trunks and click on Add SIP Trunk
Select Generic Country, Generic VOPI Provider and enter the Main Trunk Number provided.
Enter the name of the trunk. This can be anything.
For the server, provide the server name provided. (e.g. xxx.thevoip.city)
Enter the provided number of Simultaneous calls.
The type of authentication is Register/Account based.
Enter the Authentication ID - typically same as the Trunk Number.
The authentication password will be provided as well.
To ensure your calls are routed correctly, please select the extension you want to route to during and after office hours. This can be a queue or a ring group depending on your configuration.
Once you press OK on the top to save, check that the trunks are registered by looking back at the SIP trunks page. A green dot signifies that it's registered correctly.