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DTMF tones not being consistently generated by 2801

I've recently taken over management of an Asterisk system that uses a 2801 as a SIP trunk.

There's been an ongoing issue where DTMF tones entered in to external voice menu systems only work intermittently. As far as I know, everything else about the system is fine.

When I first heard of this problem I had assumed that RFC 2833 support wasn't properly enabled somewhere in the system.

I did some packet captures on the Asterisk system that show the 2833 packets coming from the phones and being passed to the 2801.

I then did another test today. I called my cell from one of the phones on this system and listened on the cell as I entered numbers on the desk phone.

I heard the first few tones, but then nothing when I hit any numbers. It's not a case of the DTMF tones being distorted; they simply aren't being generated on the 2801 in the first place.

I was running a packet capture on the Asterisk system at this time and every number I entered shows up as being passed to the 2801 as a 2833 sequence.

paolo bevilacqua
Hall of Fame Master

Update IOS.


What phones are generating the RFC2833 packets? Are you sure it is valid RFC2833 - I have seen some devices not set the marker bit in the first of the RFC2833 packets they were sending for a tone, and the 2811 we had with an ISDN card wouldn't generate the DTMF from that as it wasn't a valid DTMF event.

Cisco IP phones, mostly 7941s, originate the RFC2833 packet, though it's the Asterisk server that actually sends them to the 2801. They appear to be valid, at least Wireshark's VoIP analysis tools see them as valid and the few packets I've looked at manually seem fine.

I tried updating to 15.1(4)M5 over the weekend, but while that seems to have helped the issue of the 2801 not sending some DTMF tones it's now occasionally sending double tones. Again, a packet capture shows the 2833 packets that I'd expect, but if I listen on the other end as I dial numbers I hear about 5% of the numbers dialed twice in short succession.

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