def bulk_send_sms(request):
    """
    Sends bulk SMS to selected voters using Twilio API.
    """
    if request.method != 'POST':
        return redirect('voter_advanced_search')
        
    selected_tenant_id = request.session.get("tenant_id")
    if not selected_tenant_id:
        messages.warning(request, "Please select a campaign first.")
        return redirect("index")
    
    tenant = get_object_or_404(Tenant, id=selected_tenant_id)
    settings = getattr(tenant, 'settings', None)
    if not settings:
        messages.error(request, "Campaign settings not found.")
        return redirect('voter_advanced_search')
    
    account_sid = settings.twilio_account_sid
    auth_token = settings.twilio_auth_token
    from_number = settings.twilio_from_number
    
    if not account_sid or not auth_token or not from_number:
        messages.error(request, "Twilio configuration is incomplete in Campaign Settings.")
        return redirect('voter_advanced_search')
        
    message_body = request.POST.get('message_body')
    if not message_body:
        messages.error(request, "Message body cannot be empty.")
        return redirect('voter_advanced_search')

    select_all_results = request.POST.get('select_all_results') == 'true'
    
    if select_all_results:
        voters, _ = get_filtered_voter_queryset(request, tenant, data_source='POST')
        voters = voters.filter(phone_type='cell').exclude(phone='')
    else:
        voter_ids = request.POST.getlist('selected_voters')
        voters = Voter.objects.filter(tenant=tenant, id__in=voter_ids, phone_type='cell').exclude(phone='')
    
    if not voters.exists():
        messages.warning(request, "No voters with a valid cell phone number were selected.")
        return redirect('voter_advanced_search')

    success_count = 0
    fail_count = 0
    
    auth_str = f"{account_sid}:{auth_token}"
    auth_header = base64.b64encode(auth_str.encode()).decode()
    url = f"https://api.twilio.com/2010-04-01/Accounts/{account_sid}/Messages.json"
    
    interaction_type, _ = InteractionType.objects.get_or_create(tenant=tenant, name="SMS Text")

    for voter in voters:
        digits = re.sub(r'\D', '', str(voter.phone))
        if len(digits) == 10:
            to_number = f"+1{digits}"
        elif len(digits) == 11 and digits.startswith('1'):
            to_number = f"+{digits}"
        else:
            fail_count += 1
            continue
            
        data_dict = {
            'To': to_number,
            'From': from_number,
            'Body': message_body
        }
        data = urllib.parse.urlencode(data_dict).encode()
        
        req = urllib.request.Request(url, data=data, method='POST')
        req.add_header("Authorization", f"Basic {auth_header}")
        
        try:
            with urllib.request.urlopen(req, timeout=10) as response:
                if response.status in [200, 201]:
                    success_count += 1
                    Interaction.objects.create(
                        voter=voter,
                        type=interaction_type,
                        date=timezone.now(),
                        description='Mass SMS Text',
                        notes=message_body
                    )
                else:
                    fail_count += 1
        except Exception as e:
            logger.error(f"Error sending SMS to {voter.phone}: {e}")
            fail_count += 1
            
    messages.success(request, f"Bulk SMS process completed: {success_count} successful, {fail_count} failed/skipped.")
    return redirect('voter_advanced_search')