DTMF detection problem on inband dtmf mode

i’m facing with an issue on dtmf detection on mu solution. first of all, let me describe my topology;

  • i’m using a kamailio as load balancer having a sip trunk to a GSM/LTE network
  • behind the kamailio i have 2 asterisks installed on version 16
  • the network DTMF mode is inband. so, i’ve changed my dtmf mode from rfc2833 to inband mode in sip.conf

when i dial the IVR and input any range of digits with any speed and delay, there is no problem and asterisk detects the digits properly (for example: 123456789)
the issue is when I input duplicated digits. for example 123344567. in these cases, asterisk detects 1234567. also in some few cases asterisk ignore the duplicated digits. in my example: it detects 12567.
i’m really confused.
i appreciate if somebody help me in this case.
thank you.

Which codec are you using, e.g. DTMF will not work reliably with GSM, G.729 and other vocoder based codecs. That’s one of the main reasons for having out of band telephony event signalling.

thanks for your response. we are using g711 alaw.

It looks like there is too short a gap between digits. If you capture and analyze the audio (e.g tcpdump/wireshark/audacity), what is the length of silence between the digits?

Also, did the DTMF originate out of band (e.g. mobile phone, or VoIP)?

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