public class BillingCalloutService {
//Implement business and callout logic methods here
@Future(callout=true)
public static void callBillingService(List<Id> recordIds) {
    List<Project__c> projects = [Select Id, ProjectRef__c,Billable_Amount__c from Project__c Where Id = :recordIds];

    List<Project__c> projectsToUpdate = new List<Project__c>();

    String username = ServiceCredentials__c.getValues('BillingServiceCredential').Username__c;
    String password = ServiceCredentials__c.getValues('BillingServiceCredential').Password__c;

    String auth = username + ':' + password;
    String encodedAuth = EncodingUtil.base64Encode(Blob.valueOf(auth));

    for(Project__c p : projects) {

        BillingServiceProxy.InvoicesPortSoap11 service = new BillingServiceProxy.InvoicesPortSoap11();
        service.inputHttpHeaders_x = new Map<String, String>();
        service.inputHttpHeaders_x.put('Authorization', 'Basic ' + encodedAuth);

        BillingServiceProxy.project project = new BillingServiceProxy.project();
        project.username = username;
        project.password = password;
        project.projectRef = p.ProjectRef__c;
        project.billAmount = p.Billable_Amount__c;

        if(service.billProject(project).equals('ok')){
            projectsToUpdate.add(new Project__c(ProjectRef__c = p.ProjectRef__c, Status__c = 'Billed'));
        }
    }
    shouldIRun.stopTrigger();
    upsert projectsToUpdate ProjectRef__c;
}
}

@IsTest
 private class BillingCalloutServiceTest {

@IsTest
private static void testBillingCalloutService() {
Account a = new Account(Name = 'Acme');
insert a;
Opportunity o = new Opportunity(Name='Test', StageName='Submitted Project', AccountId=a.Id, Amount=1000, CloseDate=Date.Today());
insert o;
Project__c p = new Project__c(Status__c='Running',Start_Date__c=Date.Today(),End_Date__c=Date.Today(),Billable_Amount__c=10000,ProjectRef__c='projectX',Opportunity__c=o.Id);
insert p;

insert new ServiceCredentials__c(Name='BillingServiceCredential',Username__c='toto', Password__c='azerty');


Test.setMock(HttpCalloutMock.class, new BillingCalloutServiceMock());
Test.startTest();
  p.Status__c = 'Billable';
  update p;
Test.stopTest();
// runs callout and check results
p = [select Status__c from Project__c where id =: p.id];
System.assertEquals('Billed', p.Status__c);
}

}