public class BillingCalloutService {
    //Implement business and callout logic methods here
    //

    public List<Project__c> lstToUpdateProj = new List<Project__c>();

    @future(callout=true)
    public static void callBillingService(List<Id> projsId){
        ServiceCredentials__c sc = ServiceCredentials__c.getValues('BillingServiceCredential');
        String username = '';
        String password = '';
        if(sc!=null){
            username = sc.Username__c;
            password = sc.Password__c;
        }

        List<Project__c> lstAllProj = new List<Project__c>([Select Id,Status__c,ProjectRef__c,Billable_Amount__c from Project__c where Id=:projsId]);

        for(Project__c p:lstAllProj){
            if(p.Status__c == 'Billable'){
                BillingServiceProxy.project wrapProj = new BillingServiceProxy.project();
                wrapProj.projectRef = p.ProjectRef__c;
                wrapProj.billAmount = p.Billable_Amount__c;
                wrapProj.username = username;
                wrapProj.password = password;

                BillingServiceProxy.InvoicesPortSoap11 objBill = new BillingServiceProxy.InvoicesPortSoap11();
                objBill.billProject(wrapProj);

                p.Status__c ='Billed';
                lstToUpdateProj.add(p);
            }
        }
    }

    if(lstToUpdateProj.size()>0 && lstToUpdateProj!=null){
        update lstToUpdateProj;
    }
}

trigger ProjectTrigger on Project__c (after update) {
    //Call the Billing Service callout logic here
    //
    List<Id> lstProjIds = new List<Id>();
    for(Project__c p:Trigger.New){
        lstProjIds.add(p.Id);
    }
    BillingCalloutService.callBillingService(lstProjIds);
}

@isTest
private class BillingCalloutServiceTest {
  //Implement mock callout tests here
    static testMethod void testSuccess(){
        ServiceCredentials__c sc = new ServiceCredentials__c();
        sc.Name = 'BillingServiceCredential';
        sc.Username__c = 'bsUser1';
        sc.Password__c = 'bsPass1';
        insert sc;

        Opportunity opp = new Opportunity(Name='New Opp Test',StageName='Prospecting',Type='New Project',CloseDate=date.today().addDays(20));
        insert opp;

        Project__c proj= new Project__c(Opportunity__c=opp.Id);
        proj.Status__c = 'Running';
        insert proj;

        BillingCalloutServiceMock mock = new BillingCalloutServiceMock();

        Test.startTest();
        Test.setMock(WebServiceMock.class, mock);
            proj.Status__c = 'Billable';
            update proj;
        Test.stopTest();

        System.assertEquals('Billed', proj.Status__c);
    }

    static testMethod void testFailure(){

        ServiceCredentials__c sc = new ServiceCredentials__c();
        sc.Name = 'BillingServiceCredential';
        sc.Username__c = 'bsUser1';
        sc.Password__c = 'bsPass1';
        insert sc;

        Opportunity opp = new Opportunity(Name='New Opp Test',StageName='Prospecting',Type='New Project',CloseDate=date.today().addDays(20));
        insert opp;

        Project__c proj= new Project__c(Opportunity__c=opp.Id);
        proj.Status__c = 'Running';
        insert proj;

        BillingCalloutServiceMockFailure mock = new BillingCalloutServiceMockFailure();

        Test.startTest();
        Test.setMock(WebServiceMock.class, mock);
            proj.Status__c = 'Billable';
            update proj;
        Test.stopTest();

        System.assertEquals('Billed', proj.Status__c);
    }
}

for(Project__c p:Trigger.New){
        //add your filter logic here, so specific changes to be passed for callout
        lstProjIds.add(p.Id);
    }

List<Project__c> lstAllProj = [Select Id,Status__c,ProjectRef__c,Billable_Amount__c from Project__c where Id=:projsId];

if(lstToUpdateProj.size()>0 && lstToUpdateProj!=null){
        update lstToUpdateProj;
    }

Test.startTest();
Test.setMock(WebServiceMock.class, mock);
proj.Status__c = 'Billable';
update proj;
Test.stopTest();

//perform your DML before callout
Test.startTest();
Test.setMock(WebServiceMock.class, mock);
// check results with assets
Test.stopTest();