<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def dial(self, session_id, token, sip_uri, options=[]):
 """ Use this method to connect a SIP platform to an OpenTok session. The audio from the end of the SIP call is added to the OpenTok session as an audio-only stream. The OpenTok Media Router mixes audio from other streams in the session and sends the mixed audio to the SIP endpoint :param String session_id: The OpenTok session ID for the SIP call to join :param String token: The OpenTok token to be used for the participant being called :param String sip_uri: The SIP URI to be used as destination of the SIP call initiated from OpenTok to the SIP platform :param Dictionary options optional: Aditional options with the following properties: String 'from': The number or string that will be sent to the final SIP number as the caller Dictionary 'headers': Defines custom headers to be added to the SIP INVITE request initiated from OpenTok to the SIP platform. Each of the custom headers must start with the "X-" prefix, or the call will result in a Bad Request (400) response Dictionary 'auth': Contains the username and password to be used in the the SIP INVITE request for HTTP digest authentication, if it is required by the SIP platform For example: 'auth': { 'username': 'username', 'password': 'password' } Boolean 'secure': A Boolean flag that indicates whether the media must be transmitted encrypted (true) or not (false, the default) :rtype: A SipCall object, which contains data of the SIP call: id, connectionId and streamId """

        payload = {
            'sessionId': session_id,
            'token': token,
            'sip': {
                'uri': sip_uri
            }
        }

        if 'from' in options:
            payload['sip']['from'] = options['from']

        if 'headers' in options:
            payload['sip']['headers'] = options['headers']

        if 'auth' in options:
            payload['sip']['auth'] = options['auth']

        if 'secure' in options:
            payload['sip']['secure'] = options['secure']

        endpoint = self.endpoints.dial_url()
        response = requests.post(
            endpoint,
            data=json.dumps(payload),
            headers=self.json_headers(),
            proxies=self.proxies,
            timeout=self.timeout
        )

        if response.status_code == 200:
            return SipCall(response.json())
        elif response.status_code == 400:
            raise SipDialError('Invalid request. Invalid session ID.')
        elif response.status_code == 403:
            raise AuthError('Authentication error.')
        elif response.status_code == 404:
            raise SipDialError('The session does not exist.')
        elif response.status_code == 409:
            raise SipDialError(
                'You attempted to start a SIP call for a session that '
                'does not use the OpenTok Media Router.')
        else:
            raise RequestError('OpenTok server error.', response.status_code)