I’ve got the following setup:
PSTN —- ITSP —- SDSL Modem-Router —— Gateway —–
How are you forwarding them? Is it in such a way that you remain in the audio path, or do you get out of the audio path in the forward?
It depends on what let has the bad audio. If it’s on the SIP side
(RTP to RTP) a pcap file will show you your perspective of audio losses. Received RTCP reports should show you the other side’s perspective of audio losses as well.
If DAHDI is still in the picture in the forward scenario, that would be another place to monitor the audio.
Try to capture each leg (IP side, using tcpdump/wireshark) and on DAHDI using dahdi_monitor or something equivalent. Figure out if any of your legs of audio quality issues. If you don’t see anything, it’s something at their end.