Wireshark has IAX2 disectors, tcpdump doesn’t have jack.
app_rpt in conjunction with the DAHDI kernel module (which we are working to replace) serves as the quasi-conferencebridge system.
tcpdump is just a capture wrapper around libpcap which is the library that creates Packet CAPture files. Wireshark is the primary open source tool with the full protocol decoders built into it. There’s other stuff out there, but in the free/open space, that’s the best one to use. Copy the pcap files on your box, and move them to your laptop and read them with Wireshark.