Django formset - how to update an object?

How can I update an object from a formset using request.POST?

Here is my code and my problem is that this always creates a new PhoneNumber object. But I want to update the old PhoneNumber object.

def contact_detail(request, contact_id):
    contact = get_object_or_404(Contact, pk=contact_id)
    phone_number_list = PhoneNumber.objects.filter(contact=contact_id)

    if request.method == 'POST':
        cform = ContactForm(request.POST, instance=contact)
        #the next line is probably wrong!
        phonenumberformset = PhoneNumberFormSet(request.POST, queryset=phone_number_list)

        if cform.is_valid() and phonenumberformset.is_valid():
            phonenumber_instances = phonenumberformset.save(commit=False)
            for phonenumber in phonenumber_instances:
                phonenumber.contact = contact
                phonenumber.save()

            request.user.message_set.create(message='The contact "%s" was chanced successfully.' % contact.__str__())
            return HttpResponseRedirect("/crm/contacts/?oby=1")
    else:
        cform = ContactForm(instance=contact)
        phonenumberformset = PhoneNumberFormSet(queryset=phone_number_list)

    return render_to_response(
        'crm/contact_detail.html',
        {'cform': cform, 'phonenumberformset': phonenumberformset,},
        context_instance = RequestContext(request),
    )

Edit: I create three PhoneNumberForms:

PhoneNumberFormSet = modelformset_factory(PhoneNumber, max_num=3, extra=3, exclude=('contact',))

Edit: The solution using inlineformset_factory:

@login_required
def contact_detail(request, contact_id):
    contact = get_object_or_404(Contact, pk=contact_id)
    PhoneNumberInlineFormSet = inlineformset_factory(Contact, PhoneNumber, max_num=3)

    if request.method == 'POST':
        cform = ContactForm(request.POST, instance=contact)
        classific