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github-actions[bot] edited this page Aug 4, 2026 · 1 revision

EC20 VoLTE Setup Guide

This guide is about the modem's internal VoLTE stack — not the bridge's own. It configures the EC20 so the module registers to IMS and the bridge receives decoded PCM audio over the AT/audio path.

Since specs/015-volte-host-ims, there is a second, independent path: the bridge runs its own IMS registration over an LTE IMS PDN (volte-register), keeping SIP and media under software control. The two are mutually exclusive on one SIM — see Operations → Host-side IMS over LTE.

This guide was written for an EC20 with an Airtel India SIM; the host-side path was developed on an EC200U with a Vodafone India SIM, which is UNISOC-based and behaves differently (no QMI, AT only).

The Quectel EC20 module does not enable VoLTE by default. The ROW_Generic_3GPP MBN profile locks the IMS voice flag to 0, preventing VoLTE registration. This guide documents the steps to enable VoLTE on an EC20 module with an Airtel India SIM.

Prerequisites

  • Quectel EC20 LTE Cat 4 module
  • AT command access via /dev/ttyUSB2 (or the appropriate AT port)
  • SIM card from a VoLTE-capable carrier (tested with Airtel India, MCC/MNC 40494)

Problem

With the default ROW_Generic_3GPP MBN profile active, AT+QCFG="ims" returns +QCFG: "ims",1,0 — IMS framework enabled but VoLTE disabled. Attempting AT+QCFG="ims",1,1 returns ERROR and the second field remains 0.

Without VoLTE, the module cannot place voice calls on LTE-only networks. It must fall back to 2G (GSM) via AT+QCFG="nwscanmode",1 for circuit-switched calls, or rely on CSFB if 2G/3G coverage exists.

Solution

Step 1 — Deactivate the MBN Profile

AT+QMBNCFG="Deactivate"

This removes the carrier profile constraints that lock the IMS voice flag.

Step 2 — Reboot the Module

AT+CFUN=1,1

Wait approximately 20 seconds for the module to re-enumerate on USB.

Step 3 — Disable Echo (after reboot)

ATE0

Step 4 — Switch to LTE Only

AT+QCFG="nwscanmode",3

Restricts the module to LTE. Verify registration and signal:

AT+CREG?        -> +CREG: 0,1  (registered)
AT+CSQ           -> +CSQ: 30,3  (good signal)
AT+QNWINFO      -> +QNWINFO: "FDD LTE","40494","LTE BAND 1",390

Step 5 — Enable VoLTE

AT+QCFG="ims",1,1

The command may return ERROR, but verify the setting took effect:

AT+QCFG="ims"   -> +QCFG: "ims",1,1

Both fields should now be 1.

Step 6 — Verify

Place a test call. The call should connect over LTE without CSFB to 2G/3G.

Troubleshooting

Symptom Cause Fix
AT+QCFG="ims",1,1 returns ERROR and second field stays 0 MBN profile is still active Run AT+QMBNCFG="Deactivate" and reboot
"Out of coverage area" on incoming calls No VoLTE and no 2G/3G fallback Follow this guide to enable VoLTE, or set nwscanmode=1 for GSM-only
+CREG: 0,2 (searching) after switching to LTE No LTE coverage Switch to auto mode: AT+QCFG="nwscanmode",0
Serial output garbled after reboot Echo re-enabled on boot Send ATE0 after each reboot before issuing commands

Quick Reference

AT+QMBNCFG="Deactivate"
AT+CFUN=1,1
# (wait 20s for reboot)
ATE0
AT+QCFG="nwscanmode",3
AT+QCFG="ims",1,1
AT+QCFG="ims"              # verify: should show 1,1

Notes

  • Tested on EC20 with firmware exposing 4 MBN profiles: ROW_Generic_3GPP, OpenMkt-Commercial-CU, OpenMkt-Commercial-CT, Volte_OpenMkt-Commercial-CMCC.
  • The CMCC VoLTE profile did not enable VoLTE for Airtel — it is carrier-specific to China Mobile.
  • These settings persist across reboots. The procedure only needs to be run once.
  • Carrier: Airtel India (40494), LTE Band 1 (EARFCN 390).

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