i am using sms application for sending sms via gsm modem..after successful connection its giving me write time error while read meassage...pls help

if (receiveNow.WaitOne(timeout, false))
                    {
                        string t = port.ReadExisting();
                        buffer += t;
                    }
                    else
                    {
                        if (buffer.Length > 0)
                            throw new ApplicationException("Response received is incomplete.");
                        else
                            throw new ApplicationException("No data received from phone.");
                    }