diff --git a/pyVoIP/SIP.py b/pyVoIP/SIP.py
index e6531f1..39a84a1 100644
--- a/pyVoIP/SIP.py
+++ b/pyVoIP/SIP.py
@@ -481,7 +481,7 @@ def parseSIPMessage(self, data):
       
 class SIPClient():
   def __init__(self, server, port, username, password, myIP=None, myPort=5060, callCallback=None):
-    self.NSD = True
+    self.NSD = False
     self.server = server
     self.port = port
     self.hostname = socket.gethostname()
@@ -569,6 +569,9 @@ def parseMessage(self, message):
       debug("TODO: Add 400 Error on non processable request")
   
   def start(self):
+    if self.NSD == True:
+      raise RunTimeError("Attempted to start already started SIPClient")
+    self.NSD = True
     self.s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
     #self.out = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
     self.s.bind((self.myIP, self.myPort))
@@ -596,17 +599,19 @@ def genTag(self):
         return tag
   
   def genSIPVersionNotSupported(self, request):
-    regRequest = "SIP/2.0 505 SIP Version Not Supported\r\n"
-    regRequest += "Via: SIP/2.0/UDP "+request.headers['Via']['address'][0]+":"+request.headers['Via']['address'][1]+";branch="+request.headers['Via']['branch']+"\r\n"
-    regRequest += "From: "+request.headers['From']['raw']+";tag="+request.headers['From']['tag']+"\r\n"
-    regRequest += "To: "+request.headers['To']['raw']+";tag="+self.genTag()+"\r\n"
-    regRequest += "Call-ID: "+request.headers['Call-ID']+"\r\n"
-    regRequest += "CSeq: "+request.headers['CSeq']['check']+" "+request.headers['CSeq']['method']+"\r\n"
-    regRequest += "Contact: "+request.headers['Contact']+"\r\n" #TODO: Add Supported
-    regRequest += "User-Agent: pyVoIP """+pyVoIP.__version__+"\r\n"
-    regRequest += "Warning: 399 GS \"Unable to accept call\"\r\n"
-    regRequest += "Allow: "+(", ".join(pyVoIP.SIPCompatibleMethods))+"\r\n"
-    regRequest += "Content-Length: 0\r\n\r\n"
+    response = "SIP/2.0 505 SIP Version Not Supported\r\n"
+    response += "Via: SIP/2.0/UDP "+request.headers['Via']['address'][0]+":"+request.headers['Via']['address'][1]+";branch="+request.headers['Via']['branch']+"\r\n"
+    response += "From: "+request.headers['From']['raw']+";tag="+request.headers['From']['tag']+"\r\n"
+    response += "To: "+request.headers['To']['raw']+";tag="+self.genTag()+"\r\n"
+    response += "Call-ID: "+request.headers['Call-ID']+"\r\n"
+    response += "CSeq: "+request.headers['CSeq']['check']+" "+request.headers['CSeq']['method']+"\r\n"
+    response += "Contact: "+request.headers['Contact']+"\r\n" #TODO: Add Supported
+    response += "User-Agent: pyVoIP """+pyVoIP.__version__+"\r\n"
+    response += "Warning: 399 GS \"Unable to accept call\"\r\n"
+    response += "Allow: "+(", ".join(pyVoIP.SIPCompatibleMethods))+"\r\n"
+    response += "Content-Length: 0\r\n\r\n"
+
+    return response
   
   def genAuthorization(self, request):
     HA1 = hashlib.md5(self.username.encode('utf8')+b':'+request.authentication['realm'].encode('utf8')+b':'+self.password.encode('utf8')).hexdigest().encode('utf8')
diff --git a/pyVoIP/VoIP.py b/pyVoIP/VoIP.py
index 5852c15..ffd1429 100644
--- a/pyVoIP/VoIP.py
+++ b/pyVoIP/VoIP.py
@@ -350,7 +350,11 @@ def callback(self, request):
         self.sip.out.sendto(ack.encode('utf8'), (self.server, self.port))
     
   def start(self):
-    self.sip.start()
+    try:
+      self.sip.start()
+    except Exception:
+      self.sip.stop()
+      raise
     
   def stop(self):
     for x in self.calls.copy():
@@ -372,4 +376,4 @@ def call(self, number):
     request, call_id, sess_id = self.sip.invite(number, medias, pyVoIP.RTP.TransmitType.SENDRECV)
     self.calls[call_id] = VoIPCall(self, CallState.DIALING, request, sess_id, self.myIP, ms = medias)
     
-    return self.calls[call_id]
\ No newline at end of file
+    return self.calls[call_id]