Receiving "NO CARRIER" error while tring to make a call using GSM modem in Python

I want to make a call using my GSM modem. So I wrote the below program:

import time
import serial

recipient = "+98xxxxxxxxxx"

phone = serial.Serial("COM10",  115200, timeout=5)
try:
    time.sleep(0.5)
    phone.write(b'ATZ\r')
    time.sleep(1)
    phone.write(b'ATD"'+recipient.encode() +b'"\r')
    while(1):
        print(phone.readline())
    time.sleep(0.5)
finally:
    phone.close()

But when I run it I receive this output:

>>> ================================ RESTART ================================
>>> 
b'ATZ\r\r\n'
b'OK\r\n'
b'ATDxxxxxxxxxx\r\r\n'
b'NO CARRIER\r\n'

What does this "NO CARRIER" error means?

Note that I can send SMS successfully.

---

This is the program that I use to send SMS:

import time
import serial

recipient = "+98xxxxxxxxxx"
message = "Test"

phone = serial.Serial("COM10",  115200, timeout=5)


try:
    time.sleep(0.5)
    phone.write(b'ATZ\r')
    time.sleep(0.5)
    phone.write(b'AT+CMGF=1\r')
    time.sleep(0.5)
    phone.write(b'AT+CMGS="' + recipient.encode() + b'"\r')
    time.sleep(0.5)
    phone.write(message.encode() + b"\r")
    time.sleep(0.5)
    phone.write(bytes([26]))
    time.sleep(0.5)
finally:
    phone.close()

I found the origin of the error :

The syntax is ATD+98xxxxxxxxxx; followed by terminating string. I was forgotten to put semicolon at the end after the number.

So I replace

phone.write(b'ATD"'+recipient.encode() +b'"\r')

with

phone.write(b'ATD"'+recipient.encode() +b';"\r')

And now it works fine.

---

Based on the brackets in this documents, I thought that using ";" is optional. But it seems that I was wrong.