[SOLVED] No audio on voice call with SIM7600G-H M.2 4G HAT

Issue

I'm trying to place a voice call using a Raspberry Pi 4 and a SIM7600G-H M.2 4G HAT using minicom and AT commands. I can actually call my mobile phone and pick up the call, this workes fine. I have a pair of headphones attached to the minijack port of the SIM7600 HAT and there is a slight crackle when I pick up.

However, after that, there is no sound on the headphones whatsoever. Any ideas what I could check?

Also, I've noticed that I can play a sound using AT+CPTONE and I can hear it on the headphones before I place a call. After placing a call (without the sound working) AT+CPTONE does not play a sound anymore - but the AT command still prints OK and takes the appropriate time to run.

The command sequence I used:

AT+CPIN?
+CPIN: READY

OK
AT+CSQ
+CSQ: 23,99

OK
AT+COPS?
+COPS: 0,0,"Sunrise Sunrise",7

OK
AT+VMUTE?
+VMUTE: 0

OK
AT+CLVL?
+CLVL: 4

OK
AT+COUTGAIN?
+COUTGAIN: 8

OK
AT+CPTONE=1,1000,8000  // tone is audible on the headset
OK
ATD0?????????;  // number obfuscated, I did dial a real number
OK

VOICE CALL: BEGIN  // no audio during the call on any side
AT+CHUP
VOICE CALL: END: 000015

OK
AT+CPTONE=1,1000,8000  // tone is NOT audible on the headset anymore
OK

Any help on this is really appreciated.

Solution

Turns out, my SIM7600G-H M.2 4G HAT just had a defect. I bought another, only slightly different module and audio worked out of the box. Those few nerve-racking days of fruitless debugging could have been spent better, I guess.

Answered By - DinuTheHuman

Answer Checked By - Gilberto Lyons (WPSolving Admin)