Here is the class that I want to create a test class for

@RestResource(urlMapping='/delivery_report')
global class SmsDeliveryReport {

	@HttpGet
    global static void receiveReport() {
    	RestRequest req = RestContext.request;
        RestResponse res = RestContext.response;

        if(req.params.containsKey('Transaction') && req.params.containsKey('Status')) {
        	List<Task> updatedSms = updateDeliveryStatus(req.params.get('Transaction'), req.params.get('Status'));

        	if(!updatedSms.isEmpty()) {
        		res.statuscode = 200;
		        res.addHeader('Content-Type', 'text/html');
				res.responseBody = Blob.valueOf('REPORT_RECEIVED');

        	} else {
        		res.statuscode = 500;
		        res.addHeader('Content-Type', 'text/html');
				res.responseBody = Blob.valueOf('FAILED_TO_RECEIVE_REPORT');
        	}	        

        } else {
        	res.statuscode = 400;
	        res.addHeader('Content-Type', 'text/html');
			res.responseBody = Blob.valueOf('MISSING_REQUIRED_FIELD');
        }             
    }

    private static List<Task> updateDeliveryStatus(String transactionId, String newStatus) {
    	List<Task> sms = [
            SELECT SMS_Delivery_Status__c 
            FROM Task 
            WHERE SMS_Transaction_ID__c = :transactionId 
            ORDER BY CreatedDate DESC 
            LIMIT 1
        ];

    	if(!sms.isEmpty()) {
    		sms.get(0).SMS_Delivery_Status__c = newStatus;

    		update sms;
    	}

    	return sms;
    }
}

I've tried to test it with CalloutWithStaticResources but it turned out that the class method declared as a void type. Here is the test class that I've tried to use:

@IsTest
public class SmsDeliveryReportTest {

    @isTest static void receiveReportTest() {
        // Use StaticResourceCalloutMock built-in class to
        // specify fake response and include response body 
        // in a static resource.
        StaticResourceCalloutMock mock = new StaticResourceCalloutMock();
        mock.setStaticResource('mockResponse');
        mock.setStatusCode(200);
        mock.setHeader('Content-Type', 'application/json');
        
        // Set the mock callout mode
        Test.setMock(HttpCalloutMock.class, mock);
        HTTPResponse res = new HTTPResponse();
        
        // Call the method that performs the callout
        SmsDeliveryReport.receiveReport();
        
        // Verify response received contains values returned by
        // the mock response.
        // This is the content of the static resource.
        System.assertEquals('{"hah":"fooled you"}', res.getBody());
        System.assertEquals(200,res.getStatusCode());
        System.assertEquals('application/json', res.getHeader('Content-Type'));   
    }
       
}

The test is failed and I only got 18% coverage.
I will appreciate any kind of help. Thank you.