RM520N-GL on Windows 11: VoLTE works, uplink audio works, but no downlink/remote audio

IoT Modules 5G Module
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Hello,

Quectel technical support suggested that I post this question here for general technical guidance.

I have a Quectel RM520N-GL installed by Lenovo in a ThinkPad running Windows 11, with a Vodafone Italy SIM.

My goal is simple: make and receive normal VoLTE calls from Windows using the laptop microphone and speakers.

Module / firmware

  • Module: RM520N-GL
  • Firmware: RM520NGLAAR03A03M4G_04.220.04.220
  • Operator: Vodafone Italy
  • voice_disable=2
  • QCFG ims=1,1
  • CEREG=0,1

Outgoing calls can successfully reach the active state in CLCC.

During previous tests, microphone uplink has worked: the remote party could hear audio originating from the PC side.

The unresolved problem is the opposite direction:

I cannot obtain remote/downlink speech audio on the Windows PC.

My priority is to keep the currently installed firmware unchanged if at all possible.

I would therefore like to understand whether this is a configuration/API problem or a fundamental limitation of the RM520N-GL variant.

Main questions

  1. Does the RM520N-GL officially support bidirectional VoLTE call audio?

In other words, is there any supported configuration in which both:

  • PC microphone → remote party
  • remote party → PC speakers

work with this hardware?

  1. On Windows, what audio interface is officially supported for voice calls on RM520N-GL?

For example:

  • USB Audio Class;
  • documented PCM-over-USB;
  • Quectel host-side voice/audio API;
  • supported Qualcomm/QMI audio service;
  • external PCM/I2S codec only;
  • or another documented interface?
  1. For firmware RM520NGLAAR03A03M4G_04.220.04.220, is there any supported runtime configuration or API that exposes receive call audio without replacing the firmware?
  2. Some previous forum discussions describe RM520N-GL as effectively data-only, despite the ability to establish IMS/VoLTE calls.

Is the receive voice path intentionally unavailable on the GL variant?

If so, confirmation of that point would already be extremely useful, because it would mean that further Windows-side transport experiments cannot produce an officially supported bidirectional voice solution.

Experimental observation

For completeness, I have also experimented with a RAM-loaded audio stack.

In the latest baseline:

  • GRAPH_OPEN succeeds;
  • SET_CFG succeeds;
  • PREPARE succeeds;
  • START succeeds;
  • eight basic responses are returned with status zero.

After this, four READ-v2 requests (0x0400100B) are sent to destination instance 0x455D, using tokens 0–3.

However, I receive:

  • no READ-v2 response (0x05001005);
  • no positive PCM read;
  • no remote speech audio.

This is only an experimental setup and I am not claiming that it is an officially supported Quectel configuration.

An optional OOB diagnostic observer also caused a separate regression before GRAPH_OPEN, so it was removed from the baseline.

I have corrected GPR packet CSV and summary logs available if an engineer would like to inspect them. An earlier automated analysis used incorrect opcode offsets; the corrected analysis confirms four READ-v2 requests, not zero.

If RM520N-GL cannot do this

If bidirectional host audio is not supported on RM520N-GL, could someone from Quectel please identify an exact voice-enabled module/order code suitable for:

  • Vodafone Italy;
  • Windows 11;
  • VoLTE calling;
  • PC microphone as uplink audio;
  • remote voice reproduced on the PC;
  • preferably without an external analogue codec?

I am particularly interested in knowing whether RM520N-EU supports this exact use case, and whether it requires additional codec hardware.

A generic statement such as “VoLTE optional” is unfortunately not sufficient; I need to know the actual host audio path.

Acceptance criterion

The target is:

remote speech audible on the Windows PC + Windows microphone audible remotely, with repeatable call setup and teardown.

Any confirmation from Quectel engineering about whether RM520N-GL exposes a supported downlink voice-audio path would be greatly appreciated.

Thank you, Tom

2

Dear @Tommaso
Your firmware is a customized firmware and installed in Lenovo PC, the test is also related with PC.
It is contact with Lenovo manufacture and confirm.

3

Dear Silvia,

thank you for your reply and for confirming that firmware RM520NGLAAR03A03M4G_04.220.04.220 is a Lenovo-customized firmware.

Following your indication, I am now contacting Lenovo again and have asked them to escalate the case specifically to their WWAN/firmware engineering team.

I have also verified the Lenovo side more deeply.

The computer is a ThinkPad P16s Gen 4 AMD, type 21QR, and Lenovo Vantage reports the current Quectel package:

  • r2xwa09w
  • Quectel RM520N-GL Wireless WAN Driver
  • package version 1.3.0.38
  • release date September 3, 2026
  • status: AlreadyInstalled

Therefore this does not appear to be an outdated Lenovo driver installation.

The exact symptom remains:

  • VoLTE registration works;
  • outgoing call establishment works;
  • the call reaches ACTIVE state;
  • microphone/uplink audio from the PC reaches the remote party correctly;
  • remote/downlink call audio is completely unavailable to the host.

I understand that Lenovo owns the customized firmware integration. However, I would still appreciate clarification from Quectel on the RM520N-GL platform side.

Could you please ask the RM520N engineering team whether the Lenovo-customized GL firmware is expected to expose remote call audio through any supported host interface, such as Host PCM/QPCMV, MHI, QMI or another Quectel interface?

In particular, it would be very useful to know whether:

  1. RM520N-GL hardware/MPSS is technically capable of host-side VoLTE RX audio;
  2. this capability depends entirely on Lenovo’s customized firmware;
  3. the missing downlink is a known limitation of the GL product compared with RM520N-EU;
  4. there is any Quectel-supported firmware/API requirement that Lenovo would need to enable.

I am not asking Quectel to troubleshoot the Windows PC. I am trying to determine the exact boundary between Quectel platform capability and Lenovo firmware customization, so that Lenovo can also be given a precise technical question.

Thank you for any clarification you can provide or for forwarding this to the appropriate RM520N voice/firmware engineering team.

Best regards,
Tommaso

4

Dear @Tommaso
Sorry about this.
It is a customized project, I don’t have permission to share information.

5

Dear Silvia, I understand — and I promise I’m not trying to make you reveal Quectel’s state secrets. :grinning_face_with_smiling_eyes:Could you please give me just one tiny, non-confidential hint before this modem becomes my full-time job?

With firmware RM520NGLAAR03A03M4G_04.220.04.220, should remote/downlink voice audio theoretically be accessible by the host during a VoLTE call, or is that RX audio path available only through OEM/custom project software?

Even one of these answers would already be incredibly useful:

  • “yes, the host can access it”;
  • “yes, but only through OEM/custom software”;
  • “no, not on this project”.

And, if I’m allowed one last small question: is Host PCM / HPCM actually the right direction for obtaining RX voice audio on this project, or should I stop looking there and focus on another audio path? No confidential documents, no source code and no secret parameters — just a very small compass pointing me in the right direction. :slightly_smiling_face:

Thank you very much for your patience with this rather stubborn modem!

6

Dear @Tommaso
Sorry.
The audio design is depends on PC, I don’t know the scheme.