diff --git a/mitel_ommclient2/messages/open.py b/mitel_ommclient2/messages/open.py
index d8c25fb..a992a8e 100644
--- a/mitel_ommclient2/messages/open.py
+++ b/mitel_ommclient2/messages/open.py
@@ -4,11 +4,22 @@ from . import Request, Response
 
 
 class Open(Request):
-    def __init__(self, username, password, **kwargs):
+    """
+        Authenticate Message
+
+        Needs to be the first message on a new connection.
+
+        :param username: Username
+        :param password: Password
+        :param UserDeviceSyncClient: If True login as OMM-Sync client. Some operations in OMM-Sync mode might lead to destroy DECT paring.
+    """
+    def __init__(self, username, password, UserDeviceSyncClient=False, **kwargs):
         super().__init__("Open", **kwargs)
 
         self.attrs["username"] = username
         self.attrs["password"] = password
+        if UserDeviceSyncClient:
+            self.attrs["UserDeviceSyncClient"] = "true"
 
     @property
     def username(self):
@@ -18,6 +29,10 @@ class Open(Request):
     def password(self):
         return self.attrs.get("password")
 
+    @property
+    def UserDeviceSyncClient(self):
+        return self.attrs.get("UserDeviceSyncClient")
+
 class OpenResp(Response):
     @property
     def protocolVersion(self):
diff --git a/mitel_ommclient2/session.py b/mitel_ommclient2/session.py
index 4f69c26..81fa41b 100644
--- a/mitel_ommclient2/session.py
+++ b/mitel_ommclient2/session.py
@@ -13,6 +13,7 @@ class Session:
         :param username: Username
         :param password: Password
         :param port: Port
+        :param ommsync: If True login as OMM-Sync client. Some operations in OMM-Sync mode might lead to destroy DECT paring.
         :param connection_class: One of :class:`mitel_ommclient2.connection.Connection` or :class:`mitel_ommclient2.connection.SSLConnection`
 
         Usage::
@@ -20,11 +21,12 @@ class Session:
             >>> s.request(mitel_ommclient2.messages.Ping())
     """
 
-    def __init__(self, host, username, password, port=None, connection_class=None):
+    def __init__(self, host, username, password, port=None, ommsync=False, connection_class=None):
         self.host = host
         self.username = username
         self.password = password
         self.port = port
+        self.ommsync = ommsync
         self.connection_class = connection_class
         if self.connection_class is None:
             self.connection_class = connection.Connection
@@ -58,7 +60,7 @@ class Session:
             self._connection.connect()
 
             # Login
-            r = self.request(messages.Open(self.username, self.password))
+            r = self.request(messages.Open(self.username, self.password, UserDeviceSyncClient=self.ommsync))
 
             r.raise_on_error()