RC7620 incoming DTMF tone problem

If my RC7620 (EE SIM) dials out on 4G to my O2 phone on 4G then I can use an external DTMF chip to generate DTMF tones into the RC7620 and I can hear these on my phone (although a little distorted).
However, if I press the numeric keys on my phone to generate DTMF tones in the other direction, I do not hear them on the RC7620. DTMF reception is blocked.

If I dial out using the same SIM but on 2G then DTMF reception is OK.

Do DTMF tones work over 4G/VoLTE? Is it SIM/network dependent? If DTMF is unreliable over 4G/VoLTE then how do automated phone systems cope?

Thanks

You can see my test for decoding DTMF from peer side:


at!gstatus?
!GSTATUS:
Current Time:  266              Temperature: 28
Modem Mitigate Level: 0         ModemProc Mitigate Level: 0
Reset Counter: 1                Mode:        ONLINE
System mode:   LTE              PS state:    Attached
IMS reg state: REGISTERED       IMS mode:    Normal
IMS Srv State: FULL SMS,FULL VoIP
LTE band:      B8               LTE bw:      5 MHz
LTE Rx chan:   3775             LTE Tx chan: 21775
LTE CA state:  INACTIVE
EMM state:     Registered       Normal Service
RRC state:     RRC Connected

PCC RxM RSSI:  -53              RSRP (dBm):  -78
PCC RxD RSSI:  -87              RSRP (dBm):  -118
Tx Power:      --               TAC:         2775 (10101)
RSRQ (dB):     -10              Cell ID:     016E8A0D (24021517)
SINR (dB):      5.6

OK

+CGREG: 1,"FFFE","16E8A21",7

+CEREG: 1,"2775","16E8A21",7

AT+WDDM=1
OK
AT+WUSLMSK=00004000,1
OK
AT!MUSLEN=1
OK
atd1234567;
OK

+CGREG: 1,"FFFE","16E8A03",7

+CEREG: 1,"2775","16E8A03",7

CONNECT

+WDDI: 2     => DTMF from remote side

+WDDI: 5      => DTMF from remote side
at+vts=2
OK

+CGREG: 1,"FFFE","360A02",7

+CEREG: 1,"2775","360A02",7

+CGREG: 1,"FFFE","16E8A03",7

+CEREG: 1,"2775","16E8A03",7
at+vts=3     => send DTMF to peer side
OK

+CGREG: 1,"FFFE","360A02",7

+CEREG: 1,"2775","360A02",7

+WDDI: 6     => DTMF from remote side
ath
OK
ati3
Manufacturer: Sierra Wireless, Incorporated
Model: RC7620
Revision: SWI9X07H_00.08.24.02 f90cbd jenkins 2022/03/21 03:47:54
IMEI: 353634110012345
IMEI SV: 19
FSN: 7Q947200012345
+GCAP: +CGSM

OK

Hi jyijyi,

Thanks for that. Very useful and it works for me. I didn’t realise that AT+WDMM and the +WDDI existed as they are not listed in the latest revision of the RC76xx AT Command Reference pdf. Also the bitmask that you use is shown as “Reserved” in the pdf file too.