Selection of GSM modem for server on ESP32/C++ for group calls

What device, what GSM modem do I need to communicate with the server to make the call and what should such communication look like? I want to write a simple server in Go/C#, or it could be another language (in fact I'm thinking of putting this server on ESP32 so it will rather stand on C++) which, upon receiving a command, should make a simultaneous call to a certain group of phone numbers and listen to the response from each number (accepted/rejected), but this must be done via the mobile network, not VoIP. External service providers are out. In the case of setting up a server on an ESP32 or on a computer, communication with the modem will be via serial port (unless there is a better solution)

You can use modules like ESP32 with a FreeRTOS system to do such calling tasks and answer them independently, and then you can develop your system based on an RTOS system. Of course, you can also use such a module as part of the ESP32 system, use the serial port to communicate with it, and use the AT command or custom commands to complete the relevant task. However, such a communication module is open source, and you can develop mobile network or network applications based on such a module.
Here's a partial entry point for a test function:
cm_cmd_t cmd_vector[] = {
{"FWRITE", cm_test_write_file}, //CM:FWRITE
{"FREAD", cm_test_read_file}, //CM:FREAD
{"FMOVE", cm_test_move_file}, //CM:FMOVE
{"FDELETE", cm_test_delete_file}, //CM:FDELETE
{"GPIO_READ", cm_test_gpio_read},//CM:GPIO_READ:NUM:iomuxpin:iomuxfun
{"GPIO_WRITE", cm_test_gpio_write},//CM:GPIO_WRITE:NUM:iomuxpin:iomuxfun:value
{"GPIO_IRQ", cm_test_gpio_irq},//CM:GPIO_IRQ:NUM:iomuxpin:iomuxfun
{"GPIO_DEINIT", cm_test_gpio_deinit},//CM:GPIO_DEINIT:NUM
{"PM_REBOOT", cm_test_pm_reboot},//CM:PM_REBOOT
{"PM_POWEROFF", cm_test_pm_poweroff},//CM:PM_POWEROFF
{"PM_REASON", cm_test_pm_power_on_reason},//CM:PM_REASON
{"PM_SLEEP", cm_test_sleep_or_wakeup_enable},//CM:PM_SLEEP
{"PM_POWERKEY", cm_test_pm_powerkey},//CM:PM_POWERKEY
{"KEYPAD", cm_test_keypad},//CM:KEYPAD:5:0:1:2:3:4:5:0:1:2:3:4
{"PWM_OPEN", cm_test_pwm_start},//CM:PWM_OPEN:dev:period:period_h
{"PWM_CLOSE", cm_test_pwm_close},//CM:PWM_CLOSE:dev
{"LBS", cm_test_lbs_start},//CM:PLBS
{"HTTP", cm_test_http}, //CM:HTTP:operation
{"MQTT", cm_test_mqtt}, //CM:MQTT:...
{"ASOCKET", cm_test_asocket_cmd}, //CM:ASOCKET:...
{"VIRT_AT", cm_virt_at_test}, //CM:VIRT_AT:AT+xxxx
{"ATD", cm_demo_atd_call}, //CM:ATD
{"UPCNF", cm_demo_update_config}, //CM:UPCNF
{"KLBS", cm_demo_get_lbs_interval}, //CM:LBS
{"GETLBS", cm_kmy_lbs_start}, //CM:GETLBS
{"PIN_CMD", cm_test_pin_cmd},
{"NTP", cm_test_ntp}, //CM:NTP:operation
{"I2C", cm_test_i2c}, //CM:I2C
{"ADC", cm_test_adc}, //CM:ADC
{"SPI", cm_test_spi}, //CM:SPI
{"LCD", cm_test_lcd}, //CM:LCD
{"FOTA", cm_test_fota}, //CM:FOTA:operation
{"TTS", cm_test_tts}, //CM:TTS:operation
{"cJSON", cm_test_cJSON}, //CM:cJSON
{"FTP", cm_test_ftp} //CM:FTP
};
In addition to using the serial port for communication, there are also interfaces that support USB and PCIe.
The following function shows you how to call the group you want to call.
void cm_demo_atd_call(unsigned char **cmd,int len)
{
char at_operation[25] = {0};
snprintf(at_operation,sizeof(at_operation),"ATD%s;rn",phone_array[phone_index++]); // phone_array the group you want to call
if(cm_virt_at_init(oc_resp_cb)!=0)
{
cm_demo_printf("rnvirt_at_init:ATD-ERRrn");
return;
}
//cm_virt_at_urc_reg(oc_resp_urc_cb);
send_len = cm_virt_at_send((uint8_t *)at_operation,sizeof(at_operation));
if(send_len < 0)
{
cm_demo_printf("rnvirt_at_send:ATD-ERRrn");
cm_virt_at_deinit();
return;
}
}