When you use SagePay as your payment module with LightCheckout extension, set the phone number field as required.
SagePay extension demands the phone number specified, therefore, the phone number field must be marked as required in the LightCheckout configuration. If it is done, everything will work normally on the site of SagePay extension. But if this field is not set as required and a customer doesn’t fill this field out, the error will be returned by SagePay.
Please be aware that errors on the side of SagePay and required phone field are not caused by issues in the LightCheckout extension.