diff --git a/pushserver/applications/firebase.py b/pushserver/applications/firebase.py
index 756a6f7..10b1424 100644
--- a/pushserver/applications/firebase.py
+++ b/pushserver/applications/firebase.py
@@ -1,117 +1,115 @@
-import json
-
 from pushserver.resources import settings
 from oauth2client.service_account import ServiceAccountCredentials
 
 __all__ = ['FirebaseHeaders', 'FirebasePayload']
 
 
 class FirebaseHeaders(object):
     def __init__(self, app_id: str, event: str, token: str,
                  call_id: str, sip_from: str, from_display_name: str,
                  sip_to: str, media_type: str, silent: bool, reason: str):
         """
         :param app_id: `str` id provided by the mobile application (bundle id)
         :param event: `str` 'incoming_session', 'incoming_conference' or 'cancel'
         :param token: `str` destination device token.
         :param call_id: `str` unique sip parameter.
         :param sip_from: `str` SIP URI for who is calling
         :param from_display_name: `str` display name of the caller
         :param sip_to: `str` SIP URI for who is called
         :param media_type: `str` 'audio', 'video', 'chat', 'sms' or 'file-transfer'
         :param silent: `bool` True for silent notification
         """
 
         self.app_id = app_id
         self.token = token
         self.call_id = call_id
         self.sip_from = sip_from
         self.sip_to = sip_to
         self.from_display_name = from_display_name
         self.media_type = media_type
         self.silent = silent
         self.event = event
         self.reason = reason
         
         try:
             self.auth_key = settings.params.pns_register[(self.app_id, 'firebase')]['auth_key']
         except KeyError:
             self.auth_key = None
 
         try:
             self.auth_file = settings.params.pns_register[(self.app_id, 'firebase')]['auth_file']
         except KeyError:
             self.auth_file = None
 
     @property
     def access_token(self) -> str:
         # https://github.com/firebase/quickstart-python/blob/909f39e77395cb0682108184ba565150caa68a31/messaging/messaging.py#L25-L33
 
         """
         Retrieve a valid access token that can be used to authorize requests.
         :return: `str` Access token.
         """
         scopes = ['https://www.googleapis.com/auth/firebase.messaging']
         try:
             credentials = ServiceAccountCredentials.from_json_keyfile_name(self.auth_file, scopes)
             access_token_info = credentials.get_access_token()
             return access_token_info.access_token
         except Exception as e:
             self.error = f"Error: cannot generated Firebase access token: {e}"
             return ''
 
     @property
     def headers(self):
         """
         Generate Firebase headers structure for a push notification
 
         :return: a firebase push notification header.
         """
         if self.auth_key:
             headers = {'Content-Type': 'application/json',
                        'Authorization': f"key={self.auth_key}"}
         else:
             headers = {
                 'Authorization': f"Bearer {self.access_token}",
                 'Content-Type': 'application/json; UTF-8',
             }
 
         return headers
 
 
 class FirebasePayload(object):
     def __init__(self, app_id: str, event: str, token: str,
                  call_id: str, sip_from: str, from_display_name: str,
                  sip_to: str, media_type: str, silent: bool, reason: str):
         """
         :param app_id: `str` id provided by the mobile application (bundle id)
         :param event: `str` 'incoming_session', 'incoming_conference', 'cancel' or 'message'
         :param token: `str` destination device token.
         :param call_id: `str` unique sip parameter.
         :param sip_from: `str` SIP URI for who is calling
         :param from_display_name: `str` display name of the caller
         :param sip_to: `str` SIP URI for who is called
         :param media_type: `str` 'audio', 'video', 'chat', 'sms' or 'file-transfer'
         :param silent: `bool` True for silent notification
         :param reason: `str` Cancel reason
         """
         self.app_id = app_id
         self.token = token
         self.call_id = call_id  # corresponds to session_id in the output
         self.event = event
         self.media_type = media_type
         self.sip_from = sip_from
         self.from_display_name = from_display_name
         self.sip_to = sip_to
         self.silent = silent
         self.reason = reason
 
     @property
     def payload(self) -> dict:
         """
         Generate a Firebase payload for a push notification
 
         :return a Firebase payload:
         """
         payload = {}
         return payload