Simcom A7670G-LABE modem: Fails to hang up a call (AT+CHUP) or make a second one - Page 1
Forum section: electronics / beginners

author: Holup
author_rank: Newbie
timestamp: May 03, 2025, 04:56:03 pm
content: I am working with Simcom A7670G-LABE modem and encountering an issue where, after dialing a voice call with ATD, it is not possible to terminate the call using AT+CHUP or ATH (with AT+CVHU=0). Furthermore, after the call ends (either naturally or manually), it is no longer possible to initiate another call via ATD without resetting the modem with AT+CRESET.This behavior is consistently observed on two different prototypes with the A7670G-LABE (Global).However, prototypes based on the A7670E-LASE (Europe) module operate correctly under the same test conditions.Test setup...Modem: A7670G-LABEController: ESP32-WROOM-32D (using 3V3 <-> 1V8 level shifter)UART settings: 115200 baud, 8N1, RTS/CTS connected, hardware flow control disabled (AT+IFC=0,0)Power supply: 4.2V, stable, no brownoutsATI Output:Code:ATIManufacturer: SIMCOM INCORPORATEDModel: A7670G-LABERevision: A7670M7_V1.11.1Observed behavior:The ATD command succeeds and establishes the first voice call.AT commands like AT, AT+CPIN?, AT+CREG?, etc. continue to respond during and after the call.AT+CHUP and ATH commands timeout (after 9 seconds).It is not possible to initiate a second call with ATD - timeout (after 2 minutes).Modem reset (AT+CRESET) restores functionality and I am able to make one call again.I am able to send SMS with AT+CMGS before and after the call multiple times without reset.Sending AT+CHUP before the first call results in OK.Example 1 - Attempting to hang up during an active call:Code:15:33:40.408 -> *ATREADY: 115:33:44.706 -> +CPIN: READY15:33:45.352 -> SMS DONE15:33:46.591 -> +CGEV: EPS PDN ACT 115:33:51.167 -> PB DONE15:33:52.638 -> ATD+421949XXXXXX;15:33:52.818 -> OK15:33:52.865 -> +CLCC: 1,0,2,0,0,"+421949XXXXXX",145,"",,015:33:56.935 -> +CLCC: 1,0,3,0,0,"+421949XXXXXX",145,"",,015:34:01.562 -> AT+CHUP15:34:10.558 -> +CME ERROR: AT command timeoutExample 2 - Attempting to make a second call after the first call naturally ended:Code:15:52:44.751 -> ATD+421949XXXXXX;15:52:44.886 -> OK15:52:44.918 -> +CLCC: 1,0,2,0,0,"+421949XXXXXX",145,"",,015:52:52.076 -> +CLCC: 1,0,3,0,0,"+421949XXXXXX",145,"",,015:54:14.874 -> +CLCC: 1,0,6,0,0,"+421949XXXXXX",145,"",,015:54:14.874 -> NO CARRIER15:54:14.874 -> VOICE CALL: END15:54:27.438 -> AT15:54:27.438 -> OK15:54:30.518 -> ATD+421949XXXXXX;15:56:30.526 -> +CME ERROR: AT command timeoutAdditional notes:ESP32 in the tests was programmed to just relay communication between modem and PC, acting as a bridge.Flow control disabled (AT+IFC=0,0) to avoid interference.Sleep mode disabled on the ESP32.No UART data overflow or framing errors observed.What have I tried?Scope measurement did not show any brownouts (docs specifies maximum drop of 400mV during operation - not observed).Factory reset with AT&F and AT&W did not help.Tried to send "+++" with one second pause in communication before and after to return back to AT command mode from audio/data/transparent mode - did not help.Tried different prototype with different A7670G-LABE - did not help.Tried to contact Simcom support via e-mail - no response after a week.Any idea what is causing this or what else can I try to diagnose and solve it?Any idea where could I download fresh modem firmware and flashing utility in case Simcom does not respond to my communication?