public class Sms
    {
        SerialPort port;
        TimeSpan waitTime;
        EventWaitHandle eventWait;

        public Sms(string portName, int baudRate)
        {
            this.port = new SerialPort(portName, baudRate, Parity.None, 8, StopBits.One);

            this.port.RtsEnable = true; 

            this.port.ReceivedBytesThreshold = 1;

            this.port.DataReceived += new SerialDataReceivedEventHandler(port_DataReceived);

            this.waitTime = TimeSpan.FromMinutes(1);

            this.eventWait = new EventWaitHandle(false, EventResetMode.AutoReset);
        }

        public bool IsOpen
        {
            get
            {
                return this.port.IsOpen;
            }
        }

        public bool Open()
        {
            this.port.Open();

            return true;
        }

        public void Close()
        {
            this.port.Close();
        }

        public string GetCenterNumber()
        {
            var response = this.ExecAtCommand(@"AT+CSCA?");

            var number = string.Empty;

            if (!string.IsNullOrEmpty(response))
            {
                var reg = new System.Text.RegularExpressions.Regex("\\+86[0-9]{10,13}");

                var mt = reg.Match(response);

                if (mt.Success)
                {
                    number = mt.Value.Substring(1);
                }
            }

            return number;
        }

        public string ExecAtCommand(string commandText)
        {
            if (this.port.IsOpen)
            {
                this.port.DiscardOutBuffer();

                this.port.DiscardInBuffer();

                this.port.WriteLine(commandText);

                this.eventWait.WaitOne(waitTime);

                Thread.Sleep(500);

                var response = this.port.ReadExisting();
              
                return response;
            }
            else
            {
                return string.Empty;
            }
        }

        void port_DataReceived(object sender, SerialDataReceivedEventArgs e)
        {
            if (this.port.BytesToRead > 0)
            {
                Thread.Sleep(500);

                this.eventWait.Set();
            }
        }
    }