C# – illegal attempt to associate a collection with two open sessions fluent nhibernate

I have this exception "illegal attempt to associate a collection with two open sessions", it raises every time I save entity contains collection of children.
I google it. I found that I opened two or more sessions when calling save, but I'm sure that I'm using only one session.
Where I did wrong? How can I solve this problemn?
Note: I'm using MVC4, and fluent NHibernate.

Entities:

public class Employee : EntityBase<int>
{
    public Employee()
        : base()
    {
        Phones = new List<Phone>();
    }

    public Employee(int id) : this() { Id = id; }
    [Browsable(false)]
    public override ApprovalBase Approval
    {
        get;
        set;
    }

    public virtual string Name { get; set; }
    public virtual string Job { get; set; }

    [Browsable(false)]
    public virtual IList<Phone> Phones { get; set; }
}
public class Phone : EntityBase<int>
{
    public Phone()
        : base()
    {
    }
    public Phone(int id) : this() { Id = id; }
    public override ApprovalBase Approval
    {
        get;
        set;
    }

    public virtual string PhoneNumber { get; set; }
    public virtual string PhoneType { get; set; }
    public virtual int EmployeeId { get; set; }

    public virtual Employee Employee { get; set; }
}

Mapping:

public sealed class EmployeeMap : ClassMap<Employee>
{
    public EmployeeMap()
    {
        Table("dbo.Employee");

        Id(x => x.Id).Column("EmployeeId");
        Map(x => x.Name);
        Map(x => x.Job);
        HasMany(x => x.Phones).KeyColumn("EmployeeId").Table("dbo.Phone").Cascade.All().Inverse();
    }
}
public sealed class PhoneMap : ClassMap<Phone>
{
    public PhoneMap()
    {
        Table("dbo.Phone");
        Id(x => x.Id).Column("PhoneId");
        Map(x => x.PhoneNumber);
        Map(x => x.PhoneType);
        Map(x => x.EmployeeId);
        References(x => x.Employee).Column("EmployeeId")
            .Not.Update()
            .Not.Insert();
    }
}

Repository:

public abstract class RepositoryBase<TEntity, TIdentity>
    : IRepository<TEntity, TIdentity>
    where TEntity : EntityBase<TIdentity>
    where TIdentity : IComparable
{
    private readonly IPersistor<TEntity, TIdentity> persistor; //contains the session to operate with the database
    public IPersistor<TEntity, TIdentity> Persistor { get { return persistor; } }

    private readonly IFinder<TEntity, TIdentity> finder;
    public IFinder<TEntity, TIdentity> Finder { get { return finder; } }

    private RepositoryBase() { }

    public RepositoryBase(
        IPersistor<TEntity, TIdentity> persistor,
        IFinder<TEntity, TIdentity> finder)
    {
        this.persistor = persistor;
        this.finder = finder;
        this.finder.DataSource = Query();
    }

    // Get entity by ID
    public virtual TEntity Get(TIdentity id)
    {
        return persistor.Get(id);
    }

    /// <summary>
    /// Validate and Save the entity. If the validation failed, will not save the entity,
    /// but returns back list of error messages.
    /// </summary>
    public virtual IList<String> Save(TEntity entity)
    {
        if (entity == null)
            throw new ArgumentNullException("entity");

        IList<String> errors = entity.Validate();

        if (errors.Count == 0)
        {
            persistor.Save(entity);
        }

        return errors;
    }

    // Delete entity from persistance repository
    public virtual void Delete(TIdentity id)
    {
        persistor.Delete(id);
    }

    /// Gets IQueryable which we use from the concrete
    /// implementation of repository to implement our 
    /// query methods (FindBy).
    protected IQueryable<TEntity> Query()
    {
        return persistor.Query();
    }

    public IList<TEntity> GetAll()
    {
        return persistor.Query().ToList();
    }
}

public class EmployeeRepository : RepositoryBase<Employee, int>, IEmployeeRepository
{
    public EmployeeRepository(
        IPersistor<Employee, int> persistor,
        IEmployeeFinder entityFinder)
        : base(persistor, entityFinder) { }

    public IEmployeeFinder Find
    {
        get { return (IEmployeeFinder)Finder; }
    }
}
public class PhoneRepository : RepositoryBase<Phone, int>, IPhoneRepository
{
    public PhoneRepository(
        IPersistor<Phone, int> persistor,
        IPhoneFinder entityFinder)
        : base(persistor, entityFinder) { }

    public IPhoneFinder Find
    {
        get { return (IPhoneFinder)Finder; }
    }
}

After I fill all the information of the Employee and add collection of phones, when I press save, the information haven't been saved in the database. After some debugging, I found that when my program reaches to "Session.SaveOrUpdate(entity);" the exception above appeared.
How to solve this issue?