How to send and read sms using gsm modem with at commands

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C#

Visual-Studio

I want send and receive sms using gsm modem with AT commands.I used as sample https://www.codeproject.com/Articles/38705/Send-and-Read-SMS-through-a-GSM-Modem-using-AT-Com
but this program send message <<content not supported>> and sometimes not send message.

What I have tried:

port.PortName = "COM6";
port.ReadTimeout = 2000;
port.Open();
port.Write("AT" + "\r");
port.Write("AT+CMGF=1" + "\r");
Thread.Sleep(1500);
// port.Write("AT+CSCS=\"PCCP437\"" + "\r");
//Thread.Sleep(100);
port.Write("AT+CMGS=\"" + txtSIM.Text.Trim() + "\"" + "\r\n");
Thread.Sleep(1500);
port.Write(txtMessage.Text.Trim() + "\x1A");
MessageBox.Show("Message successfully");