SMS application using C# and GSM modem is giving error

Question

I am trying to make a SMS application to send message to phone number from this C# application by using GSM modem. but it gives this eorror.

+CME Error:PACM(AP),UNREGISTED

OK

OK

+CME Error:PACM(AP),NOT_ALLOWED_SLO

OK

+CME Error:PACM(AP),NOT_ALLOWED_SLO

OK

+CME Error:PACM(AP),NOT_ALLOWED_SLO

OK HI

ERROR

here is my code

    private void btnsend_Click(object sender, EventArgs e)
    {
        try
        {
            SerialPort sp = new SerialPort();
            sp.PortName = txtport.Text;
            sp.Open();
            sp.WriteLine("AT" + Environment.NewLine);
            Thread.Sleep(100);
            sp.WriteLine("AT+CMGF=1" + Environment.NewLine);
            Thread.Sleep(100);
            sp.WriteLine("AT+CSCS=\"GSM\"" + Environment.NewLine);
            Thread.Sleep(100);
            sp.WriteLine("AT+CMGS=\"" + txtphone.Text + "\"" + Environment.NewLine);
            Thread.Sleep(100);
            sp.WriteLine(txtmessage.Text + Environment.NewLine);
            Thread.Sleep(100);
            sp.Write(new byte[] { 26 }, 0, 1);
            Thread.Sleep(100);
            var response = sp.ReadExisting();
            if (response.Contains("ERROR"))
            {
                MessageBox.Show("Sending fialed", "Message", MessageBoxButtons.OK, MessageBoxIcon.Stop);
                MessageBox.Show(response);
            }
            else
            {
                MessageBox.Show("Message sent", "Message", MessageBoxButtons.OK, MessageBoxIcon.None);
            }
            sp.Close();
        }
        catch (Exception ex)
        {
        }


    }
}

}