get AT command call response - command

Question

I'm trying a code to make a voice call using usb modem and it succeeded to make a call... now i want to get that call response to know if number is ringing,busy or unavailable
This is my used code:
string number = textBox1.Text;
po.PortName = "COM3";
po.BaudRate = int.Parse("9600");
po.DataBits = Convert.ToInt32("8");
po.Parity = Parity.None;
po.StopBits = StopBits.One;
po.ReadTimeout = int.Parse("300");
po.WriteTimeout = int.Parse("300");
po.Encoding = Encoding.GetEncoding("iso-8859-1");
po.Open();
po.DtrEnable = true;
po.RtsEnable = true;
po.Write("ATDT "+number+";\r");
System.Threading.Thread.Sleep(7000);
po.WriteLine("ATH+CHUP;\r");
po.DiscardInBuffer();
po.DiscardOutBuffer();
po.Close();

Answer 1

After ATD, you need reading the port for kind of information called URC.
For voice call, there are the following possible response,
If no dialtone
NO DIALTONE
If busy,
BUSY
If connection cannot be set up:
NO CARRIER
NO ANSWER
And, before ATD, you'd better set the error format using at+cmee, for exam, at+cmee=2 will enable the string format.
EDIT：(Here is an example with python)
#! /usr/bin/env python
# -*- coding: utf8 -*-
from __future__ import print_function
import sys
import serial
NUM = "111111111"
ser = serial.Serial("com1", 115200)
ser.write('at+cmee=2\r')
ser.timeout = 10.0
res = "invalid"
while len(res) > 0:
res = ser.read(1)
print(res, end='')
ser.write('atd' + NUM + ';\r')
ser.timeout = 60.0
res = "invalid"
while len(res) > 0:
res = ser.read(1)
print(res, end='')
ser.write("AT+CHUP\r")
ser.timeout = 10.0
res = "invalid"
while len(res) > 0:
res = ser.read(1)
print(res, end='')
Its output is (I reject the call from the phone "111111111"),
at+cmee=2
OK
atd111111111;
OK
NO CARRIER
AT+CHUP
+CME ERROR: operation not allowed
And, after the output of 'no carrier', there is no more need to hang up.