PSIGate
From WHMCS Documentation
Setup
To activate the PSIGate module in WHMCS, navigate to Setup > Payments > Payment Gateways and choose PSIGate from the Available Gateways dropdown.
Once activated, you can then customise the name to something more friendly such as "Credit Card" and enter the Store ID Pass Phrase corresponding to your PSIGate account.
In order to use this payment gateway module some firewall adjustments may be necessary; port 7989 for test mode, and/or 7934 for live mode must be open for incoming and outgoing connections.