[HELP] No audio with SIP to ISDN (CAPI), ISDN to SIP is fine


I have some audio problems with Asterisk in combination with ISDN. I have been browsing the web and reading everything I could find for a week, without success.

I have set up Asterisk with a Eicon Diva Server BRI-2M PCI ISDN card (using CAPI), connected to a PBX. Asterisk is configured to answer a certain MSN. I also have connected a couple of Cisco 7912 SIP phones to the network Asterisk lives on.


  • SIP calls between the Ciscos work excellently (canreinvite both yes and no)
  • Calling the Asterisk box (voicemail, etc.) from both SIP and ISDN also works
  • Calling a SIP phone from ISDN works too


  • Calling an internal ISDN number or an outside line from a SIP phone works (phone rings, call can be answered) but there is no audio

Well, if I press any keys on the ISDN phone, I can hear some very faint and heavily distorted sounds from the speaker on the Cisco, but that’s all. On the ISDN phone, I hear nothing at all.

When I use a SIP phone to dial into Asterisk and then manually redirect the call to ISDN, audio works fine one way, only from SIP to ISDN. From ISDN to SIP, I hear the same faint, distorted DTMF tones when I press some keys, but again, that’s it.

Any ideas as to what could be the problem? I have been looking into the codec configurations, and I’m pretty sure that everything is on ulaw. I feel I’m out of options.