Introduction
VoLTE uses 4G (LTE) technology to transfer voice audio from one point to another using digital packets as opposed to 3G (UMTS) and 2G (GSM) networks which transfer voice audio over a dedicated analogue channel. VoLTE has many advantages which includes, less bandwidth to transfer higher quality audio than voice calls made by 3G (UMTS) and 2G (GSM) technologies. VoLTE has very much more data carrying capacity compared to 3G (UMTS) and 2G (GSM) technologies and as such can connect many more calls simultaneously.
Below are the lists of Siretta Modems that support VoLTE:
ZETA-NLP-LTE1 Ultra Low Power LTE Cat 1 (EU) Siretta Enabling Industrial IoT
ZETA-NSP-LTE1 Low Power LTE Cat 1 (EU) Siretta Enabling Industrial IoT
ZETA-NEP-LTE4 (EU) with RS232, USB serial and GPIO Low Power LTE Cat 4 European Modem with GPIO Siretta Enabling Industrial IoT
ZETA-GEP-LTE4 (EU) with RS232, USB serial, GPIO and GNSS Low Power LTE Cat 4 European Modem with GPIO and GNSS Siretta Enabling Industrial IoT
This application note guides users on how to transfer voice audio over LTE technology from a Siretta modem using the ZETA-NLP-LTE1 (EU). The VoLTE solution is simple to implement, where users make a voice call from a Siretta Modem to a Mobile phone or Land line number. Once a call is answered, voice audio, music audio or DTMF tones can be sent from the PC attached to the Siretta Modem via the USB interface to the Mobile/Landline that answers the call.
A good use case for VoLTE is in a data call centre which allows call forwarding and call waiting rules. In addition with specific mobile partner software running on the PC, the Siretta modem can be used to make voice calls in a similar way to a mobile phone device.
Equipment used for this application note:
Siretta Part Number: 61054 (ZETA-NLP-LTE1 (EU) Starter Kit (A Low Power Modem with RS232 and USB serial) Ultra Low Power LTE Cat 1 (EU)
Modem Starter Kit- Quick Start Guide ZETA-xxP Quick Start Guide
Initial Modem Setup Guide Application Note Initial Modem Setup Guide Application Note
IMS enabled SIM card with the ability to make and receive voice calls.
Mobile phone for making and receiving voice calls.
A PC with an installed Terminal Emulator program (Window 10 Laptop installed with Tera Term is used for this guide)
AT command reference manual for ZETA-NLP-LTE1 (EU) ZETA-xxP-LTE1 / ZETA-xxP-LTE4 AT Command Reference Manual
Steps:
1. Follow the Modem Starter Kit-Quick Start Guide.
2. Follow pages 1 to 5 of the Initial Modem Setup Guide Application Note.
NOTE: Make sure you use a 4G / LTE enabled SIM card that supports VoLTE (IMS) with the ZETA-NLP-LTE1 (EU).
3. Set Modem to 4G (LTE) by issuing the following AT command AT WS46=28
4. You will receive OK when modem is set to 4G (LTE) technology.
5. Check if Modem is registered to a network by issuing AT CREG?
NOTE: Modem is registered to a local network (if it returns CREG: 0,5 means it is registered to a roaming network. If you have problems with this stage please contact support.)
6. Leave default value for AT#DIALMODE=0 (Setting for voice call only).
7. Set service class as voice by issuing AT FCLASS=8
8. You will receive OK when service class is set to voice.
9. Check if device is registered with IMS using the following command AT CIREG?
NOTE: IMS is required for VoLTE and is also known as the IP Multimedia Subsystem.
10. Receive CIREG: 0,1 when it is registered to IMS, if not check if IMS service is enabled or not by issuing the following AT command AT#IMSPDPSET?
11. Also, the user can check IMS using the following AT command AT CGDCONT?
NOTE: For the ZETA-NLP-LTE1 (EU) modem the IMS service uses profile 2 as seen above.
12. Use the following AT command AT$QCPDPIMSCFGE? To confirm profile 2 is enabled.
13. The settings for ZETA-NLP-LTE1 (EU) to make a voice call is now complete.
14. Dial a voice call from ZETA-NLP-LTE1 (EU) to a mobile number 07400245346 by issuing ATD07400245346;
15. You will receive OK.
16. Mobile phone will start to ring, indicating an incoming call from the ZETA-NLP-LTE1 (EU).
17. Answer incoming call from ZETA-NLP-LTE1 (EU).
18. A voice call is established between the ZETA-NLP-LTE1 (EU) modem and the mobile phone.
19. Once a voice call is established then issue the following AT command AT VTS=123456789 from the modem, ZETA-NLP-LTE1 (EU).
20. You will hear DTMF tones from the mobile phone speaker corresponding to 123456789.
21. You have successfully made a voice call and transferred DTMF tones over LTE.
NOTE: This DTMF feature can be used to automate responses to a third party system.
Application Example:
The example below shows how the ZETA-NLP-LTE1 (EU) modem can be integrated with the standard PC application Audacity. Audacity is a free and open-source digital audio editor and recording application which can be downloaded from the following link Download | Audacity (audacityteam.org).
22. Download Audacity, depending on your operating system (Supported Window 7-10, macOS and Linux).
23. For this guide Audacity for Windows 7-10 is installed on a Windows 10 PC.
24. Connect the ZETA-NLP-LTE1 (EU) modem to a PC using a USB-A to USB-B cable.
25. Go to the PC device manager and check that the Telit USB appears under the Modems listing.
NOTE: If the Telit Modem did not appear on the device manager then download the USB drivers from Software Library Siretta Enabling Industrial IoT Software Updates or contact your Siretta representative.
26. Once the modem is connected, issue the following AT command to the Modem AT#USBCFG=11 (To enable voice over USB).
27. Receive OK when the command is accepted.
28. The modem will instantly reboot.
29. The Terminal (Tera Term) will disconnect from the COM port until reboot cycle is completed and connection is re-established.
30. Issue the following
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Introduction 
