I’ve come across this issue a few times, it seems very confusing but is an easy fix.
The scenario is:
– ISDN PBX connected to a Vega 100/200/400 gateway using a SIP provider for inbound & outbound calls.
– When ringing an inbound number that comes in over SIP, through the gateway and to the ISDN PBX, the call works fine but no ring-back is heard.
The ISDN system is sending a progress indicator message to the gateway tell it that it will send in-band media. The Vega gateway translates this into a “183 Session Progress” message which would be the SIP equivalent.
However, the PBX then clearly isn’t actually sending any audio so the caller just hears silence till the call is answered by someone.
I suspect that the reason this problem doesn’t occur on a real ISDN line is that BT et al. just ignore these messages and generate ringing whether the PBX said it was going to send it’s own or not.
So the fix is to force the Vega gateway to always send a “180 Ringing” message no matter what the ISDN side says. This tells the SIP server upstream to generate it’s own Ringing rather than expect to get some audio from the gateway.
In the Vega web interface:
- go to Expert Config
- click on SIP
- scroll right to the bottom and click Advanced SIP
- find the setting called “183 Session Progress if media present. If =2, send 183 instead of 180 with sdp”
- change it to 1 (a number one) instead of the default value of 2
Apply the changes and save the config.
That should work around the issue.