Handling SMS replies to register/unregister phone numbers
Here is my alternative. Your original code calls the DB for the same record twice (potentially) -- the get_or_create does the same as your try/except and creates the record.

def save_number(request):
    from_number = request.POST.get('From', '')
    subscribe = request.POST.get('Body', '').lower().strip()    
    obj, created = SaveNumber.objects.get_or_create(phone_number=from_number)
    response = twiml.Response()

    if created:
        response.message("Reply with stellar and you're in! :-)")
    else:
        if subscribe == "stellar":
            obj.subscribed = True
            obj.save()
            response.message("You're in. Always text outtahere, and we'll remove you.")
        elif subscribe == "outtahere":
            obj.subscribed = False
            obj.save()
            response.message("You're out")
    return response


Disclaimer: i didnt actually test it theyre may be bugs