EG915U-EU - Voice calls failing with Iliad Italy M2M SIM - Works only with standard PDP, fails with Quectel stack

IoT Modules LTE Standard Module
1

Dear Quectel Support and Community,

I am experiencing an issue with voice calls on the EG915U-EU module (firmware EG915UEUABR03A01M08) when using Iliad Italy M2M/IoT SIM (PLMN 22250).

Hardware / Software

  • Module: EG915U-EU
  • Firmware: EG915UEUABR03A01M08
  • SIM: Iliad Italy M2M (data + SMS works perfectly, voice does not)
  • For comparison I also tested a Vodafone consumer SIM (full VoLTE support)

Problem Summary

With Iliad M2M SIM:

  • VoLTE never registers (no IMS registration).
  • When using Quectel TCP/IP stack (QICSGP + QIACT), outgoing calls fail: ATD returns OK but no ringing, timeout after ~30-40s.
  • When using standard PDP context (CGDCONT + CGACT) + network auto mode, calls work via CSFB (ringing on 2G), but data suspends during the call (+QIURC: “pdpdeact”,1).
  • Trying to create a dedicated IMS bearer (CID=2, “IPV4V6”, APN “ims” or “iliad”) always fails with +CME ERROR: 3 (operation not allowed).
  • In LTE-only mode (nwscanmode=3) → NO CARRIER + +CEER: 1,1 (unassigned number).

With Vodafone SIM (same module and firmware):

  • VoLTE works reliably with standard PDP, Quectel TCP/IP stack, and hybrid configuration.

Tests Performed

I did extensive testing with both SIMs and all common configurations:

  1. Standard PDP only → Iliad: CSFB works | Vodafone: VoLTE works
  2. Quectel TCP/IP stack only → Iliad: fails (no ring) | Vodafone: VoLTE works
  3. Hybrid (stack + dedicated IMS bearer on CID=2) → Iliad: CGACT ERROR 3 | Vodafone: VoLTE works
  4. IMS enabled/disabled, LTE-only, GSM-only, factory reset, etc.

Questions for Quectel

  1. Is there a known issue with EG915U-EU (especially revision BR03A01M08) and Iliad Italy (PLMN 22250) regarding VoLTE / CSFB?
  2. Is there a specific MBN profile, firmware update, or undocumented AT command to make VoLTE or reliable CSFB work with Iliad M2M SIMs?
  3. Why does the Quectel TCP/IP stack prevent CSFB on Iliad while it works fine on Vodafone?
  4. Is there a recommended way to have permanent TCP/SSL data (stack) + reliable voice calls on the same module with Iliad M2M SIMs?

I could verify that this issue is present with more “M2M like” SIM cards.

I can provide full logs, exact command sequences, and comparison tables if needed.

Any help, firmware recommendation, or configuration advice would be highly appreciated.

2

Dear Customer,

Please refer answer as below:

1. Regarding a possible issue between EG915U‑EU (EG915UEUABR03A01M08) and Iliad Italy (PLMN 22250):
Based on our internal analysis and the test results you provided, there is no indication of a firmware defect or hardware malfunction on the EG915U‑EU module. The firmware version EG915UEUABR03A01M08 operates as expected and is confirmed to support both VoLTE and CSFB, as demonstrated by successful VoLTE operation with a Vodafone SIM on the same module. The behavior observed with the Iliad Italy M2M SIM is consistent with a network and subscription limitation, where IMS/VoLTE services are not provisioned. As a result, voice calls rely on circuit‑switched fallback, which is subject to specific data bearer conditions.

2. Regarding availability of MBN profiles, firmware updates, or AT commands to enable VoLTE or improve CSFB behavior:
At this time, there is no MBN profile, firmware revision, or AT command that can enable VoLTE or force IMS registration when the SIM or network subscription does not allow IMS services. The failure to activate a dedicated IMS bearer, including the reported CME ERROR: 3, indicates that IMS access is restricted at the SIM or network level. Since VoLTE functionality depends on explicit IMS provisioning by the operator, it cannot be enabled solely through modem configuration or firmware changes. Similarly, firmware updates cannot bypass network‑side service restrictions.

3. Regarding why the Quectel TCP/IP stack affects CSFB behavior on Iliad but not on Vodafone:
The observed difference is related to how voice services are handled by each network rather than an inconsistency in the module’s operation. With Vodafone, voice calls are handled via VoLTE, allowing simultaneous LTE data and voice without fallback, so the Quectel TCP/IP stack functions normally. In contrast, Iliad M2M SIMs do not support VoLTE and require circuit‑switched fallback for voice calls. The Quectel TCP/IP stack maintains an active LTE data bearer, which prevents the LTE suspension required for CSFB, thereby causing outgoing calls to fail. When the standard PDP context is used instead, the LTE data session is released during the call, allowing CSFB to complete successfully.

4. Regarding recommended configurations for concurrent data and voice services on Iliad M2M SIMs:
With the current Iliad M2M SIM capabilities, it is not possible to maintain permanent TCP/SSL data connectivity using the Quectel TCP/IP stack while also ensuring reliable voice service. Because VoLTE is not supported, voice calls inherently require CSFB, which necessitates temporary suspension of LTE data. The most stable and supported approach is to use the standard PDP context and accept data interruption during voice calls. As an alternative, some customers implement an application‑level workaround by deactivating the data context before placing a call and reactivating it afterward. For applications requiring simultaneous, uninterrupted data and voice services, we recommend using a SIM product that supports VoLTE or explicitly supports concurrent LTE data and voice operation.

Regards,
Fazrul Redzuan