B410p and outbound DTMF


We’re running a B410p ISDN adapter using Asterisk and the misdn drivers as loaded by zaptel-1.4.4. The adapter is connected to 3 BRI lines all in TE ptp mode.

Our handsets are all SNOM 320s which are configured to send DTMF via INFO.

Inbound and outbound calling works perfectly, but we have a problem with outbound DTMF tones not being recognised by remote parties. Transmission of DTMF is erratic - sometimes some of the tones are recognised, and sometimes they are recognised multiple time. Other times they are not recognised at all.

I’ve tried what few DTMF parameters I could find, but most of them seem to relate to inbound detection.

Is anyone else out there having similar problems, or better still, has anyone else got this working reliably?

I’m pretty sure that if I switch the phones over to inband DTMF, then things might improve - but this will then break DTMF via our VSP connections.


Hi there … i’ve got a couple of systems using b410p’s, and all Snom’s and using DTMF via INFO set to off. Although I haven’t actually had anyone mention the fact that they are having DTMF problems, I’m sure that in the last 3-4 months someone would have mentioned it by now. Ports are in TE ptmp mode.

For anyone who’s interested, I’ve found the cause of the problem.

Firstly, there are no problems with the B410p or misdn.

The issue is with the SNOM phones generating inband DTMF tones as well as the SIP INFO messages. This is causing the duplicate DTMF tones out of the ISDN channel.

I also discovered a second issue which is that the SNOM phones occasionally generate INFO messages with ridiculously long durations - Asterisk tries to treat these literally and breaks the resultant channel.

My workaround is to edit chan_sip.c to ignore durations on SIP INFO messages, and to set senddtmf=no in misdn.conf (therefore allowing the default inband tones out on their own).

I have a small DTMF pb.
during the conversation with external interlocutors, I regularly receive DTMF tones (A, # * or otherwise) without me or my interlocutor accidentally push on a key on the phone.
What can cause it?