curl --request GET \
  --url https://demo.onlinebillingform.com/api/v1/vpn/servers/list \
  --header 'token: <api-key>'

{
  "success": true,
  "vpn_systems": [
    {
      "server_name": "UK2",
      "ip_address": "192.168.2.4",
      "ssh_port": 22,
      "ssh_username": "root",
      "max_connections": 0,
      "vpn_type": "openVPN",
      "flag_countries": "NL",
      "status_online": 0,
      "vpn_details": null
    },
    {
      "server_name": "NYC",
      "ip_address": "192.168.2.3",
      "ssh_port": 22,
      "ssh_username": "admin",
      "max_connections": 0,
      "vpn_type": "openVPN",
      "flag_countries": "US",
      "status_online": 0,
      "vpn_details": null
    },
    {
      "server_name": "LA",
      "ip_address": "192.168.2.2",
      "ssh_port": 22,
      "ssh_username": "admin",
      "max_connections": 0,
      "vpn_type": "openVPN",
      "flag_countries": "US",
      "status_online": 0,
      "vpn_details": null
    },
    {
      "server_name": "UK3",
      "ip_address": "192.168.2.1",
      "ssh_port": 22,
      "ssh_username": "admin",
      "max_connections": 1,
      "vpn_type": "openVPN",
      "flag_countries": "PH",
      "status_online": 0,
      "vpn_details": null
    }
  ]
}