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G.711 is transported over RTP. RTP can be transported over UDP or TCP. But in your trace, it is transported over UDP. To analyze RTP, go to "Telephony -> RTP streams" and then select the two RTP flows and click on Analyze. In the resulting overview, you can see that there is ~3% packet loss from -> I suspect that this amount of packet-loss is enough to kill the Fax transmission, but I'm not 100% sure.

As the capture was made on (as can be seen by the frames with a length smaller than 60 bytes), there could of course also be packetloss towards the, but you would need to capture on the .1 side to be able to determine this.

Please check the switch port statistics of the involved switches to see if there are any discards and/or errors on the ports.

From the ARP requests I assume the subnetmask for is, which means would be in the same subnet, however I see that the IP TTL of packets from is 63. This indicates that there was a routing hop involved. Is a NAT address? Or is the subnetmask for misconfigured perhaps?